Joe...I'm not awesome...I just try to take some time to help people understand some of the more complex issues concerning thier bodies and try to make sure that no harm is done, especially by oneself to oneself.
Testosterone works in different ways at different ages in the male body. During puberty, testosterone accelerates the closure of the growth plates in the bones, and that's why young males who have testosterone excess or who take testosterone supplements risk ending up with "short-stature."
What you teacher says is not strictly true, although testosterone is important in building muscle mass and strength. If a male in puberty doesn't continue stretching, the muscle mass won't necessarily "stiffen." However, additional stretching and exercise of the muscles will help maintain the proper muscle function and control, which is different from muscle-flexibility.
Once past puberty, excessive testosterone (Testosterone itself, not the precursor androgens that I mentioned in the other messages.) will affect the male dancer a number of ways -- good and bad. It's "good" if it increases male vitality (increased libido), but "bad" if it causes water retention, acne, enlargement of the breasts, hepatocellular dysfunction or tumor formation, etc.
I talk about this stuff because I want the male dancers out there NOT to get involved with fads like supplemental androgens (like DHEA) that may end up damaging the body rather than helping it.
When I was more actively involved with sports medicine, the field had a great concern for the fads that went through places like gyms, sports teams, etc. One goal was to work with athletes so that they did what was proper for their bodies and not damage them. Sports medicine never got interested in ballet, and so there never was the focus on what was good and bad specifically for dancers, except by a few medical specialists. A good book of instructions is not readily available for dancers-in-training.
If you need supplements like testosterone, that decision is best made with your family physician.
And HEY! Don't keep me too busy with questions! I just realized I'm late for a class![/i]