MARSEILLE/PARIS — French teenagers blockaded some high schools and marched through cities on Thursday as the government promised to address their grievances about teacher shortages and dilapidated school buildings but gave few specific details. The protest movement, now into its third week, has exposed deep tensions in France between popular demands for investments in deteriorating public services and a cash-strapped government under pressure from bond markets to reduce its debt and deficit. Large crowds demonstrated in Paris and across the country from Lille in the north to Rennes and Nantes in the west, and Toulon and Marseille in the south. The marches were peaceful, though scuffles between police and protesters took place at some locations in Paris and Nantes and police charged at a crowd and used water cannon in Rennes. In Paris' iconic Bastille square, where a large crowd had gathered, police charged protesters and used water cannon as dusk fell. High-school teacher Diane Di Sciullo joined students at a large march in Marseille, saying all they were asking for was a decent educati