In my recent career, I've taught 6th grade (11-year-olds) up to 12th grade (18-year-olds).
If you're looking for a really good style guide, the best possible suggestion I think any person could make is The Elements of Style. It's old, but it's the absolute standard for quality writing.
If you want something that delves more into writing as a process, then I heartily recommend On Writing (though be warned, this is a guide to fiction writing; even so, King's love and respect for the use of language is very present).
Both are great books, and quite enjoyable to read.
As for your students' writing assignments, I think you'll find success if you look for examples of single-paragraph prompts and examples. 250 words is about right for a single, well developed paragraph...maybe two slightly shorter paragraphs.
Check out this: https://www.bibme.org/mla
I believe the information on citing within containers might be applicable, although I don't see why you wouldn't cite it like a normal website. I suppose it depends on what you're doing and how you apply it.