In some of my GCSE english coursework I have plagiarised by copying some sentences from the internet. Do all essays get moderated and do they use cheating checker on all of them?

If you have to submit your work to something like JISC, then obviously it means it's going to be checked for plagiarism.

Otherwise, I think it'll be down to whoever is marking your coursework. As they read through it, if a sentence seems unnatural or isn't in the type of language you'd usually write in, it's very likely your teacher will Google search the sentence at least.

If it does come up as plagiarised, he or she might then investigate the entire coursework more carefully, to understand how much of it is copied.

However, if the sentences you've included match your normal writing style, they might go under the radar.

Either way, keep it original from now on, and use quotes when you do borrow from other sources.