All the native spanish talkers....or whatever lmao! :D

I need your help!

I am a bit confused when it comes to second person in plural...
like this

¿Vosotros estudiáis? and  ¿Vosotros estudiad? 

I mean, can you even use this -ad or not in this case...and if you can, when all do you use it? I'm really confussed because I often hear my spanish prifessor say it but we learned to say it with estudiáis.... :)

Gracias in advance :D