Because there are always ways to unblock YouTube at school. However, if the reins are not going to be loosened but just all-round control, more revolt is likely to happen with students. As educational institutions, schools believe they have the responsibility to protect the generation from inappropriate pictures, profanity, or violence whatsoever that might be frequently appearing on YouTube-like websites.
It is of understandable concern to the school that students are likely going to take advantage of the privileges if YouTube was unblocked.
