A modern SUV, car, and police car are visible outside of the windows of the bus when they are riding home from skating. Also, the advertisement on the bus is for Lumber Liquidators which was not founded until the 1980s.
The short skates with no toe stops and at least the breakdancing-like moves used in the film are associated with post-roller disco "jam skating." Both came onto the scene in the '80s. Roller disco skates had high-cut boots like figure skaters and had toe stops.
When Curtis gives X his new pair of skates, the packaging says "Sure Grip" but when he opens it you can clearly see the Riedell Orange "R" symbol on the heel of the skate. For one, the two skate brands do not match up, and also, the skate is a modern Riedell model not produced in the 70s.
When Xavier opens the gift from his dad, the skates he receives are all black. However, when Xavier skates in the final skate-off competition against Sweetness, his skates have clear colored stripes on the side.