We do tux-and-drape pictures for the yearbook, but students have the chance to bring in extra changes of clothes for their additional pictures.
In general, I do suggest that you put the effort into finding something nice in a SOLID color. It'll look less dated, more timeless in future years.
If you're talking about a head-and-shoulders close-up, long-sleeved solids really are best. You want the senior's FACE to be the center of attention, and solid-colored clothing just sort of "disappears" in a close-up. This is a conservative-type picture, so I'd go with something very basic like navy or gray. White-white can be a bit stark . . . or, if you're using a white background, white-white can disappear. Pink is a nice basic for a girl, and if the background is dark, a bright pink can be nice.
On the other hand, you're probably also doing some casual pictures, and you can go a bit crazy with these:
If you're talking about a half-body or full-body shot for a girl (either indoors or outdoors), I'd go with a little sundress in EITHER a solid OR a small print. In that case, there's "enough" of the print that it doesn't detract from the picture. Don't forget a nice piece of jewelry, and either little flip-flops or bare feet.
And, as someone else suggested, do consider props. Lots of boys like to have a senior picture made with their cars. Letter jackets are also popular. Well-behaved dogs have made their pictures into more than one senior picture at my school. Artistic kids like to bring in their favorite instrument, an easel with a piece of artwork, or something else that's special to them. I remember one girl who had her picture made with a surfboard. My daughter's planning to have her picture made with her stack of Harry Potter books and her wand. (She doesn't own robes, and I won't buy them -- I think that's a bit too much.) In a few years, I know my youngest will want to have a casual picture made in her martial arts uniform -- maybe showcasing the belts she's earned over the years. BRING whatever you're thinking of; the photographer will likely have good ideas for you, and at the worst you just set the item aside.
You don't usually pay for your senior pictures 'til after you've seen the proofs, so all you have to lose is the sitting fee. I'm willing to pay the $15 fee . . . and if I'm not happy with the casual pictures, we'll go have some made elsewhere.