One way to improve the way your students treat the computers is by encouraging ownership of the equipment. You can do this by assigning computers to individual students at the beginning of the year (or quarter if you are teaching an elective). The students can then be held accountable for any damage that occurs to the computers.
It is a good idea to hold an orientation at the beginning of the term to discuss the rules for computer use in your classroom. One of the most important rules is to not allow any type of drink or food in the computer area. A spill from a soft drink can completely destroy a computer. At the orientation, you should also go over basic functions of the computer so that your students know how to properly take care of the computer.
