Admittedly I don't do a lot of glider instruction. But I think you
will find the higher tows will be useful for the 1st few flights until
they get a basic handle on towing and basic control of the airplane.
I think what you will find is that the problem areas will be the
initial tow, judgement in getting back into the pattern and of course
the approach and landing. This means they will need to do a lot of
initial tows, entering the pattern and landing. They don't need high
tows to do this. Once the student has a basic ability to control the
glider they can refine these skills on the lower tows where they
really need to practice the problem areas, but in the process the
basic flying skills should be improving as well.

just my 0.02 worth and there a lot more experienced, glider
instructors out there than I.

Brian
CFIIG/ASEL