After you've decided a dancer is ready to progress to pointe shoes, here are some suggestions for finding them the best shoes for their first pair.
First try to find a shop with a good selection of brands and models of pointe shoes as well as fitters who know their stock and dancer's feet. With new shoes coming out all of the time, it's really challenging for a teacher to keep abreast of every brand and style. Having a good selection of shoes and fitters doesn't take you out of the loop, it just saves you and your dancer some time by narrowing down options that aren't appropriate.
Once in the shop, have the dancer take off all footwear and tights. Look at her feet and notice things like toe length, amount of slope from big toe to little toe, overall breadth of the foot and height of the foot. This will help you to determine box shape, crown height, vamp and box length.
Look for any spaces between the big and 2nd toes to determine if she will benefit from using a spacer.
Ask the dancer to releve'. Is her little toe in contact with the floor? If not, try a spacer between the big and 2nd toes and see if that helps.
Look at where the metatarsal ridge is on the foot - where the toes hinge with the foot, if you choose a box that is longer than this, the dancer will be fighting the box to releve'. This doesn't mean that the vamp can't be slightly longer, but the stiffer part of the box shouldn't be longer if possible.
Have the student point her foot without weight on it, does her foot get narrower than it was when she was in the standing position? Look at the pointed foot from the side, do you see any rolling of the sides of the foot towards the sole (creating a mild to strong tubular line)? If the answer to either of these is yes, you may be dealing with a compressible foot. Look for a shoe with structured side wings and a slight to moderate taper. If you fit the shoe and it is sleek when standing flat, but you see extra material in the heel when the dancer goes on pointe, there is a good chance the dancer is sliding down into the shoe because of the compression. Try going 1 size narrower, not shorter, to see if that prevents the foot from dropping down. The shoe may feel slightly tighter then normal when flat, but there should be no crossing of the toes or pain.
Watch the shank and make sure it doesn't pull away from the sole of the foot. If it does there is a chance that the shank is too hard. If the shank is too soft, the dancer may sit in the shoe and the heel will look dropped.
If the shank is pre-arched you may find that the shoe puts the dancer off her center if the arch doesn't match hers. A progressive or 3/4 shank may provide a good option for a student with a flatter arch, but strong foot. It will allow the student to roll up onto pointe and provide a nice line to the foot.
Check the length of the box by having the dancer take a wide 2nd position and grand plie'. The toes should graze the front of the box but definately not pull back, curl or feel crammed into the front of the shoe.
Next have the dancer releve' and make sure the top of the shank does not go past the heel of the dancers foot. Check for any twisting of the shank, rolling in or out on the foot, too much space in the heel, dropping into the box, shank pulling away etc.
Make sure the box and shank are appropriate for the type of training you provide your dancer. Some shanks promote / require more spring to get onto pointe, if you do a lot of rolling up to releve' these may not be appropriate for your dancer. If you do a lot of springing onto pointe, having a slightly harder box may help the shoe last a little longer.
Most importantly, if you can't find an appropriate fit for the dancer, go to another shop that offers other options in shoes. Many brands now offer the same model shoe with a variety of shank weights which can help to find a better match for the dancer.
The above is not meant to replace a well trained and experienced fitter, but just to give you and your dancer some points to think about.