I don't know if this will help your students, but here goes. From the formal linguistics perspective, the intended question is constructed by starting with
play piano
Then you attach the interrogative pronoun 'who' as the subject
who play piano
So there's no place for another subject pronoun.
When you make it present tense and imperfective aspect, the verb structure becomes
be who playing piano
The subject 'who' raises to subject position and triggers agreement with 'be' to form
who is playing piano
It's possible that your students are misunderstanding 'who' as a complementizer instead of a pronoun. So in their incorrect sentence 'who is she playing the piano' the 'who' might be intended to correspond to 'whether' in
I wonder whether she is playing the piano
Theoretically, there's a wh-complementizer at the very top of the correct question structure, but it has no spoken content in English. It's similar to 'that' that can be left out here:
I think that she is playing the piano
I think she is playing the piano
Another possibility is that the students are attempting to form
Who is she that is playing the piano
and are trying to use a null complementizer instead of 'that' which isn't allowed in English here. As in, they are forming a phrase parallel to
I like the girl that is playing the piano (but not some other girl)
which you can rephrase without the 'that is'
I like the girl playing the piano (but not some other girl)
The students may also be simply misunderstanding the prompt: Are they supposed to ask a question about the girl's identity, or what she's doing?
Incidentally, questions in English are especially weird when they involve the subject, so I'm not surprised to see ESL students struggling with them. Among other weirdness, they don't trigger do-support:
*Who does be playing the piano
4You've selected an incorrect answer (a good pointer is that another answer has more votes). Personal pronouns (I, you he, she it, we ...) are not like normal nouns. We cannot use determiners like all, some, many with them. We cannot (usually) put adjectives before them. We cannot freely use participle clauses to modify them (that is what is happening in your example). This has nothing to do with commas. You are being fed false information by someone who is guessing the answer. Don't let your teacher friend give fake news to your student. – Araucaria - Not here any more. – 2019-01-09T23:54:07.177
3I would caution that in an exercise like this, the proper criterion is not merely whether you can or cannot use a particular word. It is more useful at this level of instruction to teach the students to speak and write in ways that are in common use and promote good communication, and to avoid obscure constructions even if they are technically correct. – David K – 2019-01-10T03:34:55.817
@DavidK - Thank you :) I appreciate everyone`s responses (very much) but I was looking for a simple answer for that very reason, in the context of these students being English language learners in a foreign country. The students are Junior High School (8th) grade students who are learning English to pass their High School exams. The answers for the exams are quite specific. I want to help but, it is an education itself, learning how English is taught in different countries...how they approach it, translate it, and structure it against their own. – Hojo – 2019-01-10T23:41:08.663
@DavidK - You are absolutely correct about the type of instruction the teachers are looking for. It has been very enlightening though to this teacher, the types of responses given. Thank you for your observation. – Hojo – 2019-01-10T23:44:18.680
1@Hojo Note that the Japanese language frequently modifies pronouns with premodifying adjectival and verbal clauses, so much so that it is often debated whether Japanese really does have a pronoun class separate from its noun class. ピアノを弾く私 (lit. piano-playing I/me) is perfectly valid as a phrase in standard and colloquial Japanese. – Michaelyus – 2019-01-11T14:55:57.550
How can we distinguish the OP case from That's me playing the piano or Who is she cutting in like that!? ? – Tᴚoɯɐuo – 2019-01-11T18:25:00.500
In "That's me playing the piano", "me" is in the objective case. "That's her playing the piano" works while "that's she playing the piano" fails. "Who is she cutting in like that?" is ambiguous. I can't begin analyzing it without first knowing what it's supposed to mean. One possibility is that it's missing a parenthetical comma. – Gary Botnovcan – 2019-01-12T13:21:37.487