anonymously track responses over a year

I am conducting a study where I want to anonymously follow students over an entire school year.I want to do this by doing 4 surveys with the class.I do not need any personal details of the respondent, I just want to link the survey that 1 person completes to the other 3 surveys that the same person completes and then compare this. The respondents are children and therefore cannot remember a unique code. The survey is also conducted via a general QR code or general link. This cannot be changed. Is there a way to do this in Qualtrics?