Originally Posted by xFullCourtTenniSx
You can't cite any extremely old players to support landing on the (initially) back foot. The reason to that being that it was illegal to leave the ground, so the front foot stayed on the ground...
The rule change to allow jumping on the serve happened way back in the 50s or 60s. However, the old school serve that the OP describes still appeared to be the de facto standard serving style for amateur and elite players in the 70s and well into the 80s. The modern serving style that Will Hamilton of FYB describes has become the standard for elite servers in the past 2 decades or so.
Sure, one can still employ the old school style. Pancho Gonzales and many others had excellent serves, even by today's standards, even tho' they left the front foot on the ground for the most part. However, I would still encourage a modern serving style as suggested by FYB and the OP's coaches.
It sounds like the OP might not be getting enough knee bend
and leg drive
for the front foot to leave the ground. If you are bending the knees, then perhaps you are not timing the leg drive properly. The legs should start to extend at the trophy phase and should be fully extended by the time the racket is at its lowest point (for the back scratch phase). This means that the upward swing (to contact) of the racket head starts immediately after the legs are fully extended -- the legs fire and then the arm & racket fire.
A serve & volleyer should still be able to use the modern style. The rear leg (right leg for righties) can still kick back as the server lands on the front (left) foot. The right leg action helps to minimize the hip/lower body rotation late in the service action (the hips should rotate earlier then the rotation should transfer to the torso/upper body). The right leg might also serve as a counterbalance as the body is launched upward and forward. After the right leg kicks back, it can then swing forward if the server wishes to follow his/her serve into the net.