Can I say,

(a) Please look at front / in front.

(b) Please look at / in front of the blackboard.
"look in front [of an object]" means look at/in an area adjacent to the object and lying in a direction that's logically "in front" of it. (It doesn't mean look at the object itself.) The object may be explicitly stated or it may be omitted and implied by context.

"look at front [of an object]" is wrong.

"look at the front [of an object]" means look at the front part of the object itself. Although "Please look at the front of the blackboard" is correct English, in practice you'd never say this except in the unlikely case where you were contrasting with looking at the back of the blackboard. Normally you would just say "Please look at the blackboard".
Site Hint: Check out our list of pronunciation videos.
In my childhood days, "Please face front!" was a common schoolroom command.
OR "Please face [the] front!" (Sometimes the "please" was omitted too.)
In the assembly ,can i say ( look at the front) to make the student stand straight in line or els...please correct me...
Teachers: We supply a list of EFL job vacancies

Yes, you can say that. That would be correct.