Static vs Dynamic
Dynamic stretching should be done before class - not static stretching. Unfortunately bad habits and bad teaching practices mean that static stretching still appears more often than anything else at the beginning of rehearsals. Dynamic stretching warms the muscle up by performing dance-like controlled movements. Static stretching (stretches held in place for 30 secs) is useful AFTER class or rehearsal, however this type of stretching fatigues the muscle so there is no point in doing it before you need to work, you'll end up getting hurt.
I don't advocate ballistic stretching due to the increased risk of injury. Ballistic stretching involves "bouncing" to push your stretch further, it's something I often see people new to stretching doing, or dancers trying to really push themselves to gain another couple of inches. The risk of hurting yourself is too high to balance out the relatively small gains on offer. Don't do it.
PNF (Proprioceptive Neuromuscular Facilitation) stretching is probably the most effective. It can produce significant long term gains in flexibility. PNF stretching requires you working with someone that knows what they're doing, otherwise you're going to waste your time and possibly hurt yourself. The basic premise of PNF is that to stretch your target muscle you perform a shortening contraction of the opposing muscle which is then followed by an isometric contraction of the target muscle. If you both know what you are doing PNF can be very effective in producing long-term gains.
Research on micro-stretching is still in it's infancy, however so far it has been shown to be very effective. The idea is to only stretch your muscle to 30-40% of it's maximum and no more; going above this activates the nervous system, which in turn causes the muscle to contract. Regularly performing low intensity stretches for 60 seconds at a time, means the muscle becomes adaptable due to familiarity with the slightly increased stretch in the muscle fibres, resulting in a lengthening of the muscle as a whole.
Any stretch held for a long period of time is a bad idea. If you're doing it for more than a few minutes it's too long, if you go as far as 10 or 20 minutes you are straining and weakening your muscle and trauma sustained while doing this is liable to shorten and stiffen your muscle resulting in reduced flexibility.