You should ensure that the build plate is a bit “sticky”. What sort of stickiness works best depends a bit on what type of filament you are using, as well as sometimes what type of printer you are using. The filament that Polar 3D sells is PLA (PolyLactic Acid) filament, which sticks well to certain brands of hair spray, to the glue in many common glue sticks, or to ordinary blue painter’s tape.
Hair spray is particularly quick and easy to apply. Aqua Net Extra Hold works well; or look for any other hair spray containing VA/crotonates/vinyl neodecanate copolymer. After applying a generous layer to a new or freshly cleaned build plate, you need only lightly “touch up” the hair spray for additional prints; for a video demonstration, see:
https://www.instagram.com/p/BJBaWwrjCwF/
As the hair spray will tend to build up over time, every so often, (every few weeks, depending on how heavily used the printer is), clean the build plate with hot water and paper towels, and then reapply a fresh generous layer of hair spray.
But if glue sticks are more convenient for your classroom, they work too, though they may result in an object sticking too well if a heavy layer is applied – try to apply a very thin, even coat to the build plate. Any PVA (Polyvinyl acetate) based glue stick, including Elmer’s washable glue sticks/Purple Disappearing glue sticks, or Scotch glue sticks, should work.
A layer of blue painter’s tape on the build surface is another way to achieve an adhesive surface, though applying it is liable to take a bit more time. (Also, sometimes an object will start pulling the tape off, at which point the rest of the build will likely be warped.) Lay down a uniform layer of tape, with no gaps and no overlapping of the edges of the strips of tape. The painter’s tape will need to be replaced when its surface becomes less adhesive, typically after about five to ten prints at the same location.
For ABS filament, the above build plate preparation approaches (hair spray, glue stick, or painter’s tape) work; in addition, ABS will also stick to PEI sheets, PET sheets, or Kapton tape, or the build plate may be spread with “ABS juice” – a slurry made from ABS and acetone (which dissolves ABS).
For printers with an acrylic or metal print bed, rather than a glass build plate, another option for aiding adhesion is to install on your print bed a sheet or film of material designed to aid adhesion, such as PEI (polyetherimide) film; BuildTak is a popular brand that works for both PLA and ABS filaments (as well as certain other types of filament). Such a layer provides adhesion and easy object removal, lasting for many prints, though it will eventually need replacement.