Are you sure you got your teacher's fingering correct ? Usually, the alternate fingering for a G chord is pinky on G of high E string, middle on B of A string, ring on G of low E string. Learn BOTH ways to make the chord. The alternate chord form makes changing from and to C and F chords (among others) more efficient. Try switching from G7 to G and back using the traditional chords, then try the same switch with the alternate form. All you have to do with the alternate is raise and lower your pinky to go from G to G7 and vice versa. It may seem awkward at first, but in the long run it's an effective weapon for your arsenal.