Tens of thousands of demonstrators are expected on the streets of Paris on Tuesday as France's powerful trade unions join high school students rallying for better conditions in a wave of protests marked by clashes with police.

Students have blocked the entrances to hundreds of high schools for 15 to 17-year-olds since late September, complaining of missing teachers, overloaded timetables, dilapidated classrooms and buildings that cannot cope with rising temperatures.