Though it is neither explained in books nor movies why other schools didn't participate in this battle. But here is a theory.
Voldemort only returned to Hogwarts because he came to know that what was Harry and his friends up to and that Harry was in Hogwarts. His ultimate aim was to kill Harry and rule the wizard world.
The battle of Hogwarts last for one or two days. And the entire school tried to defend Harry and to help him complete his quest.
There was no need for Voldemort to come at Hogwarts as it was already under control.
Also Dumbledore was busy hunting Hocruxes and didn't get time to raise awareness about this. After he was killed, the Ministry or Magic and Hogwarts was under control of Voldemort and therefore, who might have thought of fighting before may have dropped this idea to stay safe.
Since other schools had nothing to do with the war, they wouldn't have considered getting involved in that. Also what kind of motivation would their students have to help out students in a different country.
Though Dumbledore indirectly called others to help in his speech in the end of Goblet of Fire.
"Every guest in this Hall," said Dumbledore, and his eyes lingered upon the Durmstrang students, "will be welcomed back here at any time, should they wish to come. I say to you all, once again -- in the light of Lord Voldemort's return, we are only as strong as we are united, as weak as we are divided. Lord Voldemort's gift for spreading discord and enmity is very great. We can fight it only by showing an equally strong bond of friendship and trust. Differences of habit and language are nothing at all if our aims are identical and our hearts are open.