Don't worry, clashes are very common. Last year we dealt with over 300 involving about 170 different students in total.
If an exam session (i.e. am or pm) lasts more than 3 hours then one can be moved to the other session in a day and you are kept in isolation between them. During that time you can revise, eat, sleep etc. If you have more than 6 hours in a day then you can either move one to the following day or do the lot in one day and you can claim special consideration for the last one. If you move one to the next day then you and a parent have to sign to say that you will not have access to a load of stuff and you have to be collected and delivered to school. (The staying at a teacher's house went out about 15 years ago).