You'll be new to things which people in your class who did GCSE are probably familiar with BUT your teacher will acknowledge this and will probably help you out more.
It may take a bit more effort to understand stuff but I wouldn't say you're gonna feel so behind for 2 years. First few weeks will be a little difficult for you - just pay good attention in lessons, and ASK QUESTIONS - whenever you don't understand something. Even if it sounds "stupid" ask anyway it is how you will progress!