I have seen this movie about 10-20 years ago in TV. It was in color. Probably american production.

Two highschool boys break into a restricted underground facility nearby the school and discover a small machine which looks almost like a radio.

They take it to the school crafting-room and connect a car battery to it.

Suddenly a white gouse or swan appeared and the school hour is over (it had just begun) – they jumped almost an hour forward in time.

Then a lot of other things happen and i remember just the final stage:

They connected the machine to the power grid and thus opened a rift in time and space. There are warriors and creatures from all ages appearing out of this rift. Even animals like dinosaurs and warriors from past wars.

In the last part they drive with their car following the power lines on the street where the effect of the machine is traveling along towards the power plant and finally they blow up one of the power poles which then stopped the effect.