Thanks everyone for all your great advice!
I usually get help with him, but not always. Even when the instructor tries to put his bridle on, he constantly lifts his head. It's not his teeth because he apparently had them done about 2 weeks ago and that they're fine.
Whenever I see him he's always at the stall. I feel bad for him. It'd be really fun if I could take him out to graze a bit sometime. (Sometimes, after riding lessons, my friend comes up and we take him to this grassy area so he can graze for about 10 minutes and then come back.)
As many of you suggested, I also contacted my instructor about it via text messages and she called me a few minutes later saying that she'll do something about it and help me be a bit more confident with him tomorrow. (I have riding lessons at 4 PM.)
When I get anxious I feel like crying a lot, and the fact that he keeps pushing me like this is not helping.
Most of the other horses there have similar behavior to Piron but just slightly less.. sudden.
One time I was tacking up this horse called Ori and he often raised his hoof. After a while, I stopped caring and so did he. To be honest, I really enjoy riding on him but my instructor says that my heels often move around on him and that It's apparently more difficult for me and that my riding is better whilst on Piron.
I don't mind riding on either, it's just the tacking up that worries me the most.