The thing is normally you do not validate demographic questionnaires; because usually demographic information is just a part of a much larger questionnaire. The demographic/personal questions are tailor made to what do you want to know about your sample; or you could see other questionnaires that have researched similar topics and see what they have asked and the reasoning as to why they asked for that information. Sorry if I did not answer your question. 

The thing is normally you do not validate demographic questionnaires; because usually demographic information is just a part of a much larger questionnaire. The demographic/personal questions are tailor made to what do you want to know about your sample; or you could see other questionnaires that have researched similar topics and see what they have asked and the reasoning as to why they asked for that information. Sorry if I did not answer your question. 

Need to go through Ethics first, that for sure.

@Pedro J. Matos Silva : do you have any references in similar context (researched similar questionnaire)

I am sorry, but as of now I have not used a persona of my sample (although it would help a lot). But if you want to create one you need to create a questions that test shared attributes and behaviors among your students, in essence what do they all have in common that could be used as a representation of them. Keep in mind that they might share some behaviors and attributes even when there is a age and gender difference. As I have understood the best way to do this is through personal interviews as they tend to give more information that way; but if you are using a survey make sure you can later validate it as not to have erroneous information. One way one might "validate" these questions is to see similar or related questions on other surveys or questionnaires; if the same questions are being asked time and time again then they will serve as tested methods and you will not have to remake the wheel in the process. With that in mind check what characteristics a student of a persona "A" might have that make him different from students in a persona "B"; but be careful of outliers or people that do not fit into any persona as you might end up creating one or more personas that only fit one person.

Now Mr. Singh is right when he warns about the slippery situation with ethnicity, even though his reference is a guide to British demographics, the problem is the same the world over. One cannot easily generalize as people that are from one state or area might not identify with the prevailing ethnic group and might belong to a minority group and their views might be different from the dominant. group.

I am not sure if I answered your question, if I did I wish you success in your work, if I did not, well a thousand pardons for my ramblings that made you waste your time.