Do I need to have insurance for my dance studio if I have it at a high school? Or can I just have a liability waiver?
You should have your own liability insurance in addtion to a waiver drafted by an attorney. Schools often require that they be named "additional insureds" on your policy. If a dancer is injured, he or she would name both you and the school as defendants. Insurance would cover your legal defense costs. For additional protection, you should consider forming a corporation or limited liabilty company. (This is for informational purposes only and does not constitute legal advice. I am admitted to the practice of law in New York State only).
Lawsuit / Dispute Attorney
A well written waiver of liability form signed voluntarily by an adult after full disclosure of all associated risks will, under certain circumstances, give some protection to a service provider. This is not true in all states, however. And also, a waiver of liability form, even if written by an attorney, can fail to protect the service provider because it was not broadly encompassing enough to include something that later is determined a known or reasonably foreseeable risk of the activity. Also, a waiver of liability form can be deemed to be an attempt to avoid responsibility that is against some public policy that exists. Minors create even more fact-specific scenarios to varied to address in an online post.
For these reasons and others, and because the laws in whatever locale you want to do this dance studio will likely be very specific, you should contact an attorney and create an attorney client relationship to fully discuss with him or her all the particulars.
It is good practice, and indeed may be a requirement of the high school, to have insurance coverage and to declare all places in the school as well as the school and/or school district an insured (either named or additional named) too.
Contacting a local attorney to guide you through is a wise thing for you to consider.
Good luck and all the best to you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney in your state. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.