سلام the first sentence is correct. "دانش آموزند" is a shortened form of "دانش آموز هستند". "هست" is omitted. this is like "they are student" or "they're student" In informal speaking or writing "دانش آموزند" is more common, and in formal writing "دانش آموز هستند" is usually used. So you can rewrite the first sentence in formal form without changing the meaning as follows: (1) "بچه هايى كه دارند بازى مى كنند دانش آموز هستند " this sentence has the same meaning as the first sentence. but you cannot write the first sentence as follows (your phrased): (2) بچه هايى كه دارند بازى مى كنند دانش آموزان هستند because it has another meaning. the short form of the above sentence is: (2) بچه هايى كه دارند بازى مى كنند دانش آموزانند the following dialog maybe help to understand the difference between the meaning of the first (1) and second (2) sentences: aa: What do the children who are playing do? (بچه هایی که دارند بازی می کنند، چه کاره هستند) bb: The children who are playing are student. (بچه هایی که دارند بازی می کنند دانش آموزند /دانش آموز هستند.) ------------ aa: Where are the students? (دانش آموزان کجا هستند؟) bb: The children who are playing are the students. (بچه هایی که دارند بازی می کنند دانش آموزانند /دانش آموزان هستند.) please inform me if I have English grammatical mistakes. and a question: what is the difference between "if I have English grammatical mistakes" and "if I make English grammatical mistakes"

Thank you Mary. I understood from your comment that there is no difference in meaning between the two sentences, and that بچه هايى كه دارند بازى مى كنند دانش آموزان هستند is more formal and only used in written expression.

سلام !!! دانش آموزند مخفف دانش آموز هستند است و در معنی تفاوتی با هم ندارند . دانش آموزند در گفتگو های عامیانه استفاده می شود و دانش آموز هستند کتابی می باشد .