I contracted with a client and the contract included 6 months of free support after releasing first version.
The client is requesting more features (of course I'm getting paid), but because of school I can't do them.
Can I give the remaining parts to someone else? If I can, should I tell the client? What if the new freelancer makes trouble? Am I responsible for his troubles?
Be clear with your client, inform him about the situation and propose to him a contract transfer to some other freelance that is able to complete the tasks and support.
Or if you want to keep control over the product that you developed, hire a freelancer youself and make him complete the requested jobs, while you will do only quality check and keep contact with your client.
As a freelancer you have committed to only supporting existing features. What you do beyond that is up to you. You can just say you are not available for further feature requests, or too busy, or give them a quote that is very high and they will find someone else.
Personally, I think you should find the time. I bet you could if you really tried. Also give yourself plenty of time in terms of the timescales you give the customer, explain that you cannot start immediately as you have other commitments.
Finding and giving the work to a 3rd party is more complicated than it might at first seem. You might get lucky or it might go horribly wrong. I would avoid that if you can.
This commercial work is going to stand you in a very good position when it comes to job hunting or future work. I really would reconsider turning it down if I were you.
PS what Mario said above is absolutely right. Sort out what you want to do and be straight and up front with the customer.
