Please help me, how do I get through this?

I'm so grateful for any help! I'm a 15 year old (boy) and I just changed course from GCSE 'O' Level to IB. In the beginning of the year, I competed for my school in a speaking competition with an audience of 3000 people and the guest of honour was the President of the National Law Society. For this reason, I was held up in lessons and my grades have deteriorated from 80% to some going below 50%. My teachers don't support this and they don't like me. They act in awful ways in class, picking on me to ask incredibly hard questions and giving me horrible punishments for the slightest misdeameanour. For example, in my school, coming late 5 times earns you a permanent discliplinary record. My teacher tried to give me one for coming late twice. My parents had to step in. :(( My final problem is that my teachers are not excellent this year. My subject coordinator once confided to me in private that he acknowledged that I had poor teachers. I don't know how to move forward. Bad grades, teachers hate me! I'm sorry to have to ask such a long question but if you can, I appreciate any comments. I'm supremely grateful to all of you who answer. Please, and thank you so much. =)