Term in English for short physical exercise in the middle of a lesson - Физкульминутка

I think you might be looking for the term "brain break":

A brain break is a break from whatever kids are focusing on. Short brain breaks during work time have been shown to have real benefits. They reduce stress and frustration and increase attention and productivity. (1)

In the classroom, brain breaks are quick, structured breaks using physical movement, mindfulness exercises, or sensory activities. Brain breaks can be done individually (like deep breathing) or as a whole class (like a round of Simon Says). (2)

understood.org (1) & (2)

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A brain break is a short mental break that is taken during regular intervals during classroom instruction. Brain breaks are usually limited to five minutes and work best when they incorporate physical activities.

thoughtco.com

I’m not aware of any established term, but “(short) exercise break” is a clear term that needs no definition. Here are some examples in use:

• Classroom Exercise Breaks for Elementary Students

• One to two exercise breaks, held daily in school, will contribute significantly to the goal of a total of 30 minutes of activity each day for every child. — Active Breaks (download)

• Short exercise breaks during class improve concentration

Alternatively, I have also seen the following used, less commonly:

• Movement break
• Fitness break (seems to indicate a bit more rigor)
• Physical activity break (a bit wordy and slightly formal)

The term “brain break” is close but the activities it covers don’t all involve exercise (such as mindfulness).

Exercise break. Or minute if you will.

English is relatively resistant to merging words ad hoc.