hypothesis:

Students' grades are correlated with their registration numbers (the order in which they registered.) The first to register had planned their schedule out in advance, are more attuned to deadlines, and more likely to be interested in Spanish (rather than just taking it to fulfill a requirement.) The last people to register don't mind missing class, because some of them missed up to 2 weeks before joining! I may test this at the end of the semester- I have 81 students, and I get to know all their registration numbers.