If the two actually made contact, but it was slight:
While running, the boy bumped into his form teacher.
or
While running, the boy knocked into his form teacher.
or
While running, the boy stumbled into his form teacher.
More contact:
The boy banged into his form teacher.
The boy slammed into his form teacher.
The boy smacked into his form teacher.
(Poor Teacher!! Get out of the way!
)
Note: The words "bumped into" can also be used casually in other contexts where it means "met up with" by accident or chance.
I was downtown yesterday and bumped into two friends of yours.
or
"ran into"
I was at the market and ran into my form teacher.
Joe(I am so glad I bumped into you. )Nation