The Cart Fan (DTS-CFAN) should fit securely in its mounting bracket but still be fairly easy to remove. For instance, when placed properly in the center bracket on the mounting plate, the attachment should be secure enough that you could lift the dynamics cart off the track by pulling up on the Cart Fan. However, if students run a Cart/Cart Fan at full speed down a 1.2 m track and let it collide with an end stop, the Cart Fan will likely be ejected from the mounting bracket. (The Cart Fan should withstand such a tumble just fine.)
There are ways of installing the Cart Fan that are less secure. The Cart Fan has molded ridges on the body. Those ridges are intended to help seat the Cart Fan in the mounting bracket. If it is not properly positioned or seated, the Cart Fan will be fairly loose.
While you may be able to operate the Cart Fan when it is not properly positioned or seated, it will be more likely to fall out of the mounting bracket.