They may have missed half a year of material in required courses that they would have taken in the second semester under BS, and they may needlessly repeat half a year of material for courses already taken. Choirs, bands, and other musical activities require regular, year-round involvement. Kevin Meidl's national survey on the effect of block scheduling on music provides valuable information on this topic. Sports programs face similar challenges. Many courses such as mathematics, science, and foreign languages are most effectively taught and learned in small, regular doses that the student can readily assimilate, rather than in large chunks.
Teachers are more likely to teach courses outside their expertise.
In fact, Canadian studies have shown that Block Scheduling definitely hurts academic performance when assessed through objective tests. The purported benefits claimed by Block gurus Robert Canady and Joseph Carroll require much further analysis and confirmation by hard measurement before they can be accepted at face value. If the course is a challenging, content-based course like mathematics or foreign language, catching up will be extremely difficult or impossible for the student. Of course, since the actual total amount of material covered in a day on the Block will be no greater and perhaps even less on the average than under traditional scheduling, the problem of missed classes seems to diminish.