Take a break and try again in a few weeks. It definitely sounds like he is not ready. I am going to bet that he does know how stressed you are about it as well- kids are great at sensing such things. It's partly a power struggle on his part, but also partly him just not being ready.
Ask his school how strict they are about the "must be trained" policy. Some are very strict, some are not. My son's prek will allow children in with Pull-Ups on the first few weeks and they make an effort to have a potty routine a few times each day for all kids. Most kids who are partly trained or needing more time, etc. will catch on at school and get into a routine and want to do as their peers- so it actually helps them to become fully trained. Not all schools are willing to do this, but some do. It's worth asking how strict the policy is.
I feel your frustration as my son wil be 3 in Sept and has ZERO interest in the potty. He has sat on it and peed a few times, but nothing consistent and every other time I put him there or suggest it he screams, so we stop. I haven't tried in weeks. I know he's not ready. There is NO way I could imagine forcing him to be ready in just a few weeks- he wouldn't do it. Nor would I expect him to.
My older son potty trained on his own when he was ready and self initiated it. He asked for the potty one day and told me "no more diapers" and that was it. It totally caught me off guard. I know that every child is different, but I am waiting for my younger son to do something similar- even if not as clear cut. I really believe that kids will train only when they are phsyically and emotionally ready.
Take a break for a few weeks and mention NOTHING of the potty. Put all of the potty stuff, underwear, etc. away. Try to reintroduce it gently in a few weeks and he might respond better. Once he is responsive, it will be SO much easier.
GOOD LUCK!