Hey Zack,
We don't require permission forms at my branch or in my system. The main thing that I would suggest is to just be selective with which games you have regularly available. Since your teen space is unique to your library, you probably have a different policy about kids joining in for gaming programs than others would. Our switch is only available during certain times and with a very restrictive audience to make sure that there's minimal issues about which games are being played.
Hope this helps!