Graduation is an formal occasion where you want to wear well-made (and age-appropriate) attire that is also comfortable, since the ceremony will be held during the summer. You also need to follow your school's dress code, such as requiring a tie for the guys and banning strapless dresses for the girls.
Male students can wear brown or black dress shoes, but make sure they have a thick sole for good support. Wear matching light-weight socks to help keep cool. Females, stick with a pair of very low-heeled pumps for better support.
A spring suit of light-weight material is appropriate for boys. You aren't expected to wear the jacket due to the weather, but bring it so you can be photographed in the full outfit before donning your gown.
Males can wear a pair of dress pants with a dress shirt if you don't have a suit. The slacks should be a solid color, like brown, black or gray. Have them properly hemmed so that they fall just above the top of your shoes.
Girls should wear dresses or skirts with a blouse in seasonal colors (light hues or pastels) and materials, like silk and linen. Since you'll have pictures taken without your graduation gown, the skirt should cover your knees---in keeping with the formality of the occasion. Wear a half-slip so the garment can lay properly and panty lines are covered.
Any dress shirt, preferably white, that a male wears should have a collar and be a solid color. If the shirt isn't part of suit, short sleeves can be worn. A good rule for a girl's blouse would be to choose one you would wear to a wedding or dinner at a high-end restaurant.
Minimal jewelry matches the mature look of your outfit. A few quality pieces---like a necklace, birthstone earrings, a matching ring on one hand and a watch on the other wrist---are suitable for either gender.