I've learnt from my stepmum that one should pray first Subh and then Fajr if one wakes up after the Fajr-prayer and after Churuq (sunrise). If one wakes up at Fajr-time you should pray first Fajr and then Subh.
I wonder if this a right way to do so or maybe I need to find a way to wake up before Churuq? The reason why I pray Fajr sometimes later is because of the short time interval between the Isha-prayer and the Fajr-prayer (4 hours). I need to sleep at least 6 hours straight otherwise I won't be productive in my school work.