You are not Mr. Miyagi and this is not television.
Get in touch with a lawyer. You need to get insurance to protect you from consequences should a kid get hurt - a waiver might work, but it won't work as well. You also need to consider that any commercial property you use is also going to face liability issues, should one of your students (or you) get injured on the property, without *them* being covered, either by your insurance or their own.
Get in touch with a lawyer, get a loan and do this right.