NOTE: borax is toxic in high amounts. Do not eat (or allow pets to eat) this ornament! Best to keep in the classroom and hang up as a decoration. 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. (3/4 cup borax in 4 cups water)
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. It usually only takes an hour or so for many crystals to start forming. Leave overnight or over a weekend for best crystal growth in all cups. Often, borax crystals start to form on the side of the cup rather than the pipe cleaner - that's OK.
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. near holiday lights or in direct sunlight. On close inspection you can 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.