My 16 year old twins have moved with their father (non-custodial) to Colorado until June and want to come back to California to finish their senior year. Their father wants me to give him custody of the boys until then. I have 100% physical custody at this point. What should I do?

Go back to court and modify the current custody order. If your case is in California then your going to go to mediation and if you both agree then it can go into writing that they are to be returning in June.

Without some type of formal way of doing it the it can turn into an absolute nightmare.

Does he have partial legal custody or visitation? Did you agree to them moving to CO?