With regard to creating spells, as a student, you would need to check the spell with the admins first, to make sure it is within the sort of ability your character would have, and be able to create. Creating new spells, while not impossible for students (Snape did it, apparently) is pretty difficult, so really it counts as a 'special power'.
Professors don't generally have this limitation, since they need to be able to teach new spells in class - and obviously, we want to encourage students to attend class in-game, and learn new spells that way.
As for house elves, students don't have personal house elves at school, so I'm not sure what you mean there. You are free to interact with (and create basic details of) the house elves at the school, as you can also do with, for example, paintings. So long as they aren't actually a separate character, you can even control what they do, within reason - since they will, by and large, do whatever you tell them anyway.
Of course, if your character has a house elf at home, you're pretty much free to control them, too, when you're out of school. However, the school doesn't let you bring them with you (if Draco didn't have one, it's a fair bet nobody does)!
