There are a few points I'd like to share that might be helpful.
1)With toddlers, It's really important to know what usually happens in the second or so before the aggressive behavior and 1 second or 2 after the aggressive behavior.
You want to identify the most typical triggers and consequences (reenforcement sources i.e. you mentioned negative attention).
For example, does the target behavior (aggression) happen most often interacting with other kids? Are there toys or preferred activities involved? Do they always get the teacher's or parent's attention or just some times?
What reduces or gets rid of aggression without reinforcing the behavior? For example, change of activity, setting etc? Are there shared activities in which aggression is not likely to happen? Why? What's makes the difference and how can you harness this?
2) It's important to describe and understand in simple, physical, play by play terms, what the specific aggressive behavior or behaviors look like.
Is it pushing, hitting, yelling, grabbing all of the above? Are some of these more specific behaviors linked to specific consequences (getting a toy from a peer; getting "negative-attention" etc) or to specific activities?
What would a more healthy behavior look like each situation? Is there a specific social skill that would be a good replacement that you can teach and then cue and saturate with praise and a celebratory song or sticker etc (immediate positive reinforcement)?
The best behavioral parenting interventions for toddler aggressive behavior, are usually "front-end" and don't miss intervening in a single aggressive behavior instance, until the behavior is reduced to acceptable levels and/or replaced with more appropriate, pro-social alternatives.
The natural positive consequences take over once the new behavior is established. For example, the teachers reduced stress from trying to stop the aggressive behavior leads to more smiling and positive social attention, in place of the negative attention.
Front-end means removing triggers (distraction, change of activity etc). It can also mean teaching and systematically reinforcing a positive replacement behavior.
Here's a great, front-end "social skills" (pro-social replacement behaviors) development resource for young kids. It can be used at home even though it was designed to be used at PS, DC and KG etc (it may be a great way of connecting practically with the teacher also). You can usually find it in your local library: Skillstreaming - Early Childhood: A Guide for Teaching Prosocial Skills - Product Detail
Believe it or not, really basic social skills can be shaped and reinforced in toddlers, so they get what they want or need without being aggressive.
It's best to teach and practice the new social skills in a way that's really, really fun. Then when you cue for them, your far more likely to see them when it counts!
The emphasis should be on positive reinforcement for a desired alternative behavior, although immediate negative consequences for negative behavior periodically, can increase the effectiveness of the positive discipline.
It's really important to be 100% consistent for a week or until the behavior is changed. You want to cut the aggressive behavior off from any form of reinforcement.
There are often "extinction-bursts" (behavior gets worse before it goes away) at first when reinforcement is cut off. But this is made much faster and easier to manage when you're reinforcing an alternative behavior, that the child knows how to do from fun learning.
One of those behavioral things most parents don't get is that intermittent reinforcement (the negative behavior gets it's intended payoff evey 10 or 20 times), is incredibly powerful at keeping problem behaviors going. The extinction-burst never really gets to end.
So if you intermittently let the behavior happen without your intended consequences once every 10 times, it's likely the 10 times you do intervene will have little or no effect.
In fact your positive efforts can actually jump over and "reinforce" the negative aggression because he got away with it once or twice.
I'd keep it positive and practical with the teacher. The last thing you want is a defensive shut down, particularly if the teacher's own responses are inadvertently maintaining or intensifying the behavior.
I've found it really helpful when working with teachers of kids in all age groups to get them to do really simple behavior observation and charting.
If you give them the tools (check out behavior charts for toddlers via google) and make sure the extra work load is low, it actually helps the teacher to focus on important behavioral details. It can really stimulate the teacher's own natural intervention, insight and positive influence on problem behavior. DC teacher are very busy you know.
At the very least you can definitely reduce/replace the aggressive behavior at home. Any positive behavior you shape at home can also help to reduce problem behavior at DC.
With toddlers, behaviors like this are usually very environment specific, so you can reliably change the behavior at home even if it persists at school.
It's always better when a problem behavior takes place in just one rather than 2 or more major environments (i.e. just at school and not at home or Vs-Vs).
If you do some replacement social-skills development at home, it would be very easy for the teacher to cue your son to transfer the home learning to the DC setting.
Teacher-behavior like toddler-behavior, will also change when the results of their efforts are beneficial and reinforcing. Reduced child-aggressiveness in the classroom has that effect most of the time.
Detailed information (i.e. charting, precise behavioral descriptions etc) from having a more structured approach like this, is incredibly helpful to any specialist you might see if the behavior doesn't change through an environmental intervention. It can really help in pinpointing any medical issues that may be contributing factors.
The majority of toddler aggressiveness problems do respond very well to a behavioral parenting intervention, even when temperament is a factor.
Finally, when it comes to social aggression and social skills development, early intervention like this can really have amazing positive consequences on your child's mental, physical, social and economic well-being, for the rest of their lives.