Use proper grammar, not txt tlk for pages and role-playing. If you struggle with spelling, use spell-check. Your writing and grammar don't have to be flawless, everyone makes mistakes, but you need to at least make a effort. "A lot" being written as 1 word, and the word "I" being written in lowercase will not be tolerated.
You can use whatever dialect of English you speak. The "official" dialect is British English (the English of Europe and the original dialect of the Harry Potter books) and should be used on public pages.
It's school full of teenagers, some swearing is only realistic. And it's ok. Just stay within reason, and only swear when it's needed. Actually write it though, don't replace it with **** or !%$@#, write it out in proper spelling with all the true letters.
Things like "Oh my god" or "That's so gay" are 100% prohibited though. Nothing that is especially offense to people of a certain religion, sexuality, ect. is ever allowed.
Characters are welcome to swear in their native languages at well. Just make sure you follow the same rules; all the rule that apply to swearing in English apply in other languages.