Well, I do hope you have no limit to this whole "drawing your characters for practice thing", because, you know, drawing can get pretty tiring.
So! Here's my characters.
Nathan: Black hair (adorably mussed up), dark brown (mahogany) eyes, edgy (that's the key word here. edgy.), school uniform (white polo and black slacks), and maybe a sweater. You can go crazy with whatever sweater design you want.
Kara: Black hair (medium length, let down, like those Palmolive models), lighter brown (think dark caramel) eyes, school uniform (think white blouse, and khaki colored skirt)
Dante: Black hair (cut short. think this), kind of nerdy. School uniform is the same as Nathan.
Kat: Black hair (in ponytail), introverted and sorta angsty, with hoodie. You can pick the design. School uniform is the same as Kara.
It's alright if you want them individual, or in one group portrait. You choose.
Good luck! Don't burn yourself out! Your health/school/work is important, ya know!