How can I easily differentiate between slang and casual speech?

I’m currently writing my college essays, and I’ve read that the best way to write is using everyday language. However, I’ve also heard to stay away from slang and obviously anything profane.

How can I tell if something is slang or just casual speech?

The specific example which brought me to write this post was that I wanted to say “a battalion of children, led by a smathering of supervisors.” “Smathering” (in this case just being my preferred way of saying “a few”) isn’t recognized as a real word (the only dictionary it’s in is the Urban Dictionary), however I hear it commonly enough (by adults, not just teens), but would that cross the line into slang?

In this case, you might be able to use the actual word “smattering” (as in a smattering of applause) and sidestep the issue entirely.