District council members negotiated with the police to leave the area.
Mask-wearing secondary students in Kwun Tong marched and blocked roads, while Tin Shui Wai MTR closed due to a fight.
The legislation calls for an annual report that the city has retained enough autonomy from China to keep its special trading rules.
Primary and secondary schools will reopen, but kindergartens and schools for children with disabilities will remain closed until Sunday.
Chief Executive Carrie Lam says police were told to let minors go home after having personal details recorded.
Dozens slip through police lines during the night by using ropes to descend from a footbridge to a highway and ride away on motorcycles
Lawyers against the ban argued that it gave Chief Executive Carrie Lam Yuet-ngor unrestricted powers to make laws.
Only students and staff with valid identification will be allowed to enter the premises.
Protesters also pledged to block public transport services in an attempt to divert police manpower.
A huge fire broke out on strategic footbridge while an armoured vehicle was hit by petrol bombs.