Polish students marched or rallied in at least six cities on the anniversary of free speech riots in 1968. The demonstrations were remarkable for their peacefulness and for the restraint shown by police. Brief disturbances broke out at the end of marches in Wroclaw and Warsaw, but only members of radical fringe groups were involved, and they were shrugged off as provocateurs by fellow demonstrators. Contributing to the mood was the announcement Tuesday that Solidarity leader Lech Walesa and other opposition negotiators have won government agreement to legalize the Independent Students Assn. after a seven-year ban. In Warsaw, police helped direct traffic around two separate marches held with official permission.