I am sure that the school has a valid concern for wanting to
protect its reputation; after all, how would parents think of the
school if their students were to be walking everywhere in public
places instead of being kept in school or at home? It is this
public perception that students left outside would engage in
nefarious activities that leads the conservative elements
pressurise the schools to do more to "keep our young off the
streets for their own good".
However, what your school has chosen to do has gone too far:
restricting one's movement outside is akin to treating the student
like a prisoner on home detention: he isn't free to move about as a
normal human being.
What the school could have encouraged was to allow the students to
change to civilian clothes, and go do what they want to do. After
all, the school should have no right to interfere with the
student's live after official school time. In fact, whenever I went
out with my school mates after school, we made sure to change at
least our shirts. That way, we could avoid any awkward encounters
with any of the teachers we met along the way, and also gives us
leeway to get ouselves out of trouble in case we happened to get
into some incident. Not that we would deliberately do so
anyway!