Classroom audio systems are a lot more complex than you would image. In classroom audio systems you have a very large room in which the sound must be evenly distributed. This causes a lot of noises from other electronics that can get mixed in with your classroom audio.
Computer audio cards have been known to produce higher frequency noise which can convert to cause audible noise in your classroom audio system. It is important that your classroom amplification has special filters to filter these sounds out. Conference audio systems are very similar to classroom audio systems as they usually have the same type of electronics that can interfere. v