Is "Meri behen kal school gaya" correct?

Why

Karan is checking word order and word choice, which is a sensible first pass — but वाक्य-शुद्धि (sentence correction) also requires checking AGREEMENT between words, not just their individual correctness or position. As learned earlier, Hindi past-tense verbs change to match the subject's gender — 'गया' is masculine, 'गई' is feminine. The subject here is 'मेरी बहन' (my sister), a feminine noun, so the verb must match: 'गई', not 'गया'. Every individual word can be spelled correctly and placed in a sensible order, and the sentence can still be wrong if the verb doesn't agree with its subject — this is exactly the kind of error वाक्य-शुद्धि practice is meant to catch. Test it: 'मेरा भाई कल स्कूल गया' is correct (masculine subject, masculine verb); swap in 'बहन' without changing the verb, and the agreement breaks. His mother is right: the verb must match the subject's gender — 'मेरी बहन कल स्कूल गई' is the correct sentence.