'periods of classes' is correct and usable in written English.
It is most commonly used to refer to a certain amount of time that classes will be taking place (e.g. during a school day, semester or year). You could use it in a sentence like: "We have 8 periods of classes each school day."
A further survey of the timetables of students of other majors in this university also shows that they have equally tight basic-course arrangements in their first two years and slightly less tight schedules which encompass more academic and professional courses in their later years and which show on average about 5 periods of class learning per weekday.
The girl who turned down the boy went home after two periods of class.
The plan was to gather south of the school and stand there, silently, until the end of the first period of classes.
British audiences (and Anglophile culturati the world over) would be familiar with the heady, sexy feeling from that period of class structures in flux.
Partial closures may also allow schools to continue providing meals or other social services during the period of class dismissal [ 86], and continue to hold social events such as proms, and academic gatherings such as SAT or ACT testing services.
Color coding is a great way to keep organized: 1 color for each period of class!
The maximum sleep periods of rtPS Classes I and II are 7 and 12 frames, respectively.
During the study, a policy of consecutive mixing and cycling periods of three classes of antipseudomonal antibiotics was followed in the unit.
For the duration of the study, a policy of consecutive mixing and cycling periods of three classes of antipseudomonal agents (meropenem, ceftazidime/piperacillin-tazobactam and ciprofloxacin/levofloxacin) was implemented in the study unit.
