They are far less necessary than most novices realize. It's not enough to hear a crashing sound. Then we hear the crashing sound. Your students can do whole plays without complicated sound effects. Experiment with simplicity.
Most effects can be the simple mechanical variety handled at the microphone.
Recorded effects are more complicated. Learn by doing. Every space has its own ambience. The sound of the human voice has a different presence in an office from that in a city bus. When my students imagined a mini traffic helicopter reporting on congestion in the school hallways, they recorded and repeated the middle of a sound effect of a passing helicopter.