How do I get out of doing this....?

Just don't go, simple as. Tell your teacher that sadly piano is no longer one of your priorities and you don't feel you will enjoy it. No one can force you to go. However, if you have left it last minute to drop out then you should actually honour your promise to be there. It would be unfair if you threw their programme plan out. If you have enough time for them to replace you then be honest with them.