Block scheduling was a double edged sword. At my HS science gained about 15 minutes per week (ooh, impressive), but the quality of the education itself remained the same, and/or probably degraded a bit as a result of the change. Prior to block scheduling my school had 8 classes spread across 7 time slots. Mondays you went to 1-4 before lunch and 6,7,8 after lunch, tuesdays were 5,1,2,3 with the same afternoon, wednesdays: 4, 5, 1, 2 and so on until friday which was 2, 3, 4, 5. The schedule wasn't idea, and block scheduling allowed us to have only 2 different days instead of 5.
The other benefit, as I already mentioned, was an marginal increase in science time.
There were more problems with scheduling classes you wanted with block scheduling though. Seniors involved with band cannot take AP biology (which I wanted to take) because of schedule conflicts. There are other examples, but I can't think of them right now (I've been out of HS for 3 years now).
well, I lost my train of thought, I'll comment again when I remember what I was saying.