I would suggest first you should run this by your insurance person, partly to make sure you are covered for this group, and partly as they may be able to give you some feedback. Do you have a person to whom you go for business/legal advice? That can be very helpful as you build your business. There are a lot of standard forms for liability out there, and they are cheap and usually quite adequate, but with minors it is really good to understand issues of liability.
Generally the parent or guardian will be the one who will have to sign.
You will also need to have policies on whether they are allowed to participate with/without a parent and with/without training. I would say most of the places I have worked that work with a lot of adolescents use a tiered system (certain things are open to them progressing upward as they get older), and requires anyone under a certain age to buy and participate in some sort of training program: gym etiquette, proper form, overtraining, etc. There are a lot of advantages to this. You generate income. You protect yourself as you are being very careful of making sure they are ready before you train them. You get a chance to get to know them and can refund their money if you sense that they are not yet ready to participate in your program.
But whatever your rules are I think it is especially important to have them spelled out in your waiver before starting to work with someone.
I have found adolescents in some ways harder to work with than younger kids, but it may be just that I click more with the elderly. It is true that the adolescent brain, as well as the adolescent body is different than that of an adult, and you may find some do well in (or are more comfortable in) groups of their own age range, and others do fine in a mixed age group.
You might want to read about the 2 deep policy and child safety guidelines of the boy scouts. Dealing with tons of adolescents (and the unfortunate side effect that places where there are tons of adolescents sometimes bring out predators) has made them create very well designed policies.
Definitely a group it is good to try to reach, and one that when you can make connection you can really make a strong lifelong impact for.