"Two math classes" in Engpsh can be referred to as "two periods of mathematics" or simply "two math periods". The term "period" is used in educational contexts to refer to a scheduled block of time during which a specific class or lesson takes place. The word "period" comes from the Latin word "perīodium" which means "a complete revolution of a heavenly body" or "a fixed and recurring interval of time", which is fitting for the use of the word in an educational context as classes typically take place at regular intervals.
Examples:
"I have two math periods back-to-back today." (我今天有两节连续的数学课)
"I'm taking two periods of mathematics this semester." (我这个学期有两节数学课)
"I have two math classes on Mondays and Wednesdays." (我周一和周三有两节数学课)