Is the sentence grammatically correct How could we check that weak students were not just repeating after strong ones?

The sentence is grammatically correct. It's well-structured and clear.

However, you could consider these minor tweaks for even smoother flow:

* "How could we have checked" - This suggests an action that was not taken in the past, making the sentence more nuanced.

* "Whether weak students were simply repeating" - This emphasizes that the students might be repeating, but it avoids the potentially negative connotation of "just".

Here are a few revised options:

* "How could we have checked whether weak students were simply repeating after strong ones?"

* "How could we have verified if weak students were merely copying from strong ones?"

* "How could we have determined whether weak students were simply parroting the answers of stronger students?"