OK, so we're being deliberately evasive.
It's not a wind instrument/doesn't have a mouthpiece. Which eliminates flute, clarinet, recorder, fife, penny whistle or trumpet.
If it's inexpensive, it can't be a violin or guitar.
It can't be a piano, keyboard, cello or drum kit as they aren't portable and inexpensive.
It's a ukulele, isn't it?
Seeing the state of the work ones after children have compulsively fucked with the machine heads, there is no way on earth somebody else's is being forcibly removed to indulge another's parent.
If it isn't a ukulele and it is a guitar or violin (the latter is likely if it's SoundStart), the ones 'supplied' by the school or music service are paid for. They aren't free. Even if the parent doesn't personally hand over the money, the school has paid for individual instruments per child - not communal ones for any random whose mother forgot it to demand.
You're still in the wrong, even with the process of elimination half answers.