Summer Intensives offer a chance for increased flexibility. After your first morning class, you are partially warmed up for the rest of the day. That is, unless you are resting in between classes in highly air conditioned environments. I recommend not to do that. A cool but not cold place, perhaps shady outdoors somewhere, is better.
Also, allow your ballet shoes and pointe shoes to dry as much as possible in between classes, they will last longer, and will not lose that exactly right fit so soon. Having two pairs of each helps, if you can do that.
Intensive training in ballet means intensive use of the flexor muscles. Battment tendu, grande battment and developpe en avant mean heavy use of the iliopsoas (hip flexor) muscles. Without constant stretching, this tension will compromise your turnout, as the tension at the side of the hips will counter the thigh's ability to rotate outwards. It will also lessen the flexibilty of the low back and front of the hip, in doing an arabesque.
A standing lunge done in between exercises will relieve the tension building up in the hip flexors and postural muscles. Finding exactly the right balance between strength and stretch is what creates power in your work.
One of the best ways to stretch for a good arabesque is at the corner of the studio where you can hold on to one barre, while placing yourself in your ideal arabesque position with your working leg on the barre of the other wall behind you. If there is a lower barre, use it so as to get an upright back position. Do a demi plie repeatedly, holding the position well-placed.
If there is no corner with barres, get a fellow student to hold your hands to keep you upright, and place your leg on the barre behind you to do your demi plies.
A wonderful stretch regimen for dancers is yoga. My favorite is "Ali McGraw - Yoga Mind & Body". It is a few years old but still available. It is not for beginners, but dancers will love it. The positions are easy for most dancers, and give fantastic relief to muscle tension. Done in the evening it will leave you stretched and ready to sleep.
A more active stretching routine is the "Classical Stretch" series. On a lighter class schedule day, or on a no-class day, the "Athletes' Intense Stretch" will get rid of the muscle tension while still allowing muscle recovery.
If you are recovering from injury, both of the above may be helpful, but please consult with your doctor, teacher or trainer as to whether you are ready to do these routines.
Losing electrolytes and dehydration can cause muscle tension and cramps. Real sea salt on your foods, calcium/magnesium supplements and "All 12" cell salts are a great help. Celery is one of the saltiest foods you can eat, organic, multiple mineral salts, and it is a hydrating food too - a perfect snack in between classes.
The apparency of weight loss through dehydration is a seductive trap. Recognize it and don't worry about weight. If you feel puffy from drinking water, then your mineral balance is off and your cells are floating in water but are not able to use it. So you're still dehydrated. Forget the junk food sports waters. Better to mix a pinch of sea salt into your water and drink it. Neon colors and a couple of minerals won't help.