He is now in Junior Middle Three . [He is now in Grade 9]

He is now in Senior Middle One [Grade 10] / Senior Middle Three [Grade 12] 

大学生=Undergraduate. First year undergraduate; second year; third year; Final year.

She is now in primary three. [Grade 3] 

He will enter the primary school next year. He will enter the Junior Middle School [Grade 7] next year.

He should be in the Senior Middle One [ Grade 10] next year. He should be in the university next year.

He is now in Junior Middle Three . [He is now in Grade 9]

He is now in Senior Middle One [Grade 10] / Senior Middle Three [Grade 12] 

大学生=Undergraduate. First year undergraduate; second year; third year; Final year.

She is now in primary three. [Grade 3] 

He will enter the primary school next year. He will enter the Junior Middle School [Grade 7] next year.

He should be in the Senior Middle One [ Grade 10] next year. He should be in the university next year.