NOTE: borax is toxic in high amounts. Do not eat (or pets eat) this ornament! The borax solution that the students are handling is fine, though washing hands after the activity would be prudent.
Each student makes a shape out of the long pipe cleaner, small enough to fit in their cup without touching the bottom or sides.
Ask them to include a little hook so that their shape can hang from a half-pipe cleaner laid across the top of their cup.
When the shapes are ready, make the borax solution:
Either make it in each cup: put 3 tablespoons of borax powder in a cup, then fill the cup with recently boiled water and stir to dissolve the powder.
Alternatively, make one large batch of borax solution, and divide among cups.
Likely not all the borax will dissolve. That is OK.
As soon as the borax solution is in a student's cup, they can lay the half-pipe cleaner across the top of the cup, then hang the pipe cleaner shape from it, so that their pipe cleaner shape dips into the borax/water solution (but does not touch the sides of the cup).
Leave the cups on a shelf where it will not be disturbed.
As the borax/water cools, box-shaped crystals of borax form on the pipe cleaner. This usually only takes an hour or so, but can be left longer. Occasionally, borax crystals start to form on the side of the cup rather than the pipe cleaner - in this case, remake borax solution for this cup and hang the same pipe cleaner shape in it.
Once enough crystals have formed on the pipe cleaners, wash out the cup, and hang the pipe cleaner shape in the empty cup again, so that the borax crystals can dry.
The borax crystals sparkle in a bright light e.g. on a Christmas tree (hung high away from little hands). Sometimes you can even see the flat faces on the bigger crystals of borax.
It still worked with old borax that left a white suspension when dissolved in the water.