Is this Sexual Abuse? What will happen if I tell someone?

I am a 17 year old girl and swim competitively for a swimming club. A boy who I have known for a long time started messaging me on facebook. I thought he was just being friendly so I replied and we had quite a long conversation, then he started to say things like 'we could shower together after swimming' I though he was joking so I didn't say no, he then got much more explicit and realising he was being serious I backed out completely and told him I only wanted to be friends and I didn't want to have this kind of a relationship with him. He was upset about it and so we agreed we'd have a hug as friends when we next saw each other.

He had other plans for the hug though, we hugged and he didn't let go and dragged me into the cubicle and started kissing me. I pulled away and managed to get out. He then texted 'sorry xxxx' and I didn't reply. A few training sessions later I was coming out of the girls showers and he was waiting for me and pulled me into the cubicle and this time shut the door and went much further and pulled down my swimming costume. I kept saying no and trying to pull away and tried to open the door but he was much stronger than me and so I couldn't stop him. I eventually managed to get away and later he then texted me 'sorry please reply xxxx' but I didn't reply again.

Is this sexual abuse and what will happen if I tell someone or how do I stop him from doing it again?

There is a teacher at school who is helping me with stress over school work and my parents don't have a great relationship and I self harm a bit so she helps me with that too. If I tell her about this to help me deal with it, not for her to do something about it will she tell someone else?