Usually, when school reopen, that's the time the English teacher
will start drowning you with Situational writing resources, model
articles, tips, and the marking scheme.
18/30 is consider average. You may have left out some important
points in your writings, structure of the essay is not clear, and
average is the choice of vocab you use, and a few grammar
errors.
A strong writing is a one with very clear structure, able to
communicate information in writing fluently, and ability to use
vocab words, with little or no major grammatical errors.
I find it a good way to practice writing situation essay by
"pretending to be eye witness".