Judging from the meaning of the question, in the first category, the twin sisters of the freshman are in a car, and the remaining two in a car are of different grades. Choose two of the three grades as C23=3, and then choose another student from the selected grades as.

C 12？ C 12 = 4, so there are 3×4= 12 species.

In the second category, the twin sisters of freshmen are not in the first carriage, so choose two students of one grade from the remaining three grades in the first carriage, that is, C 13 = 3, and then choose one of the remaining two grades as C 12? C 12 = 4, then * * has =3×4= 12 kinds.

According to the principle of classification and counting, * * there are 12+ 12=24 different riding modes.

So the answer is 24.