Let me tell you why this is a bad idea. It's true that 99% of the time the blood type of a student is going to be consistent with that of their presumed biological parents. But what if it's not? That could mean that a child is adopted and the child may not know or not want that information to be public. It could also mean that the child's biological father (or mother) is not the same person they call "Mommy" or "Daddy." Is the trauma associated with that discovery worth the benefits of the experiment?
I have a blog post on The Genetics of ABO Blood Types. It's quite popular and every few weeks I get a letter from a distraught parent who has just discovered that the blood type of their children doesn't match the blood type of the parents. Here's the latest example (posted with permission) ...
Dear Professor Moran: I hope you don't mind my writing to you, but I just came across your blog, Sandwalk after doing some research about blood types and wondered if you might have an opinion....If high school teachers aren't knowledgeable enough to handle these situations then they should avoid these "experiments."
My daughter, in 7th grade is working on a blood type project at school and came home quite upset yesterday after telling the teacher that she was type A+ and both her Dad and I are O+. The teacher (whom I have not yet dragged over hot, burning coals....) told her that that was impossible - that she would either have to have been adopted or have a genetic defect.... It got me thinking that perhaps I had made a mistake somewhere along the way and we spent some time last night digging through info to try to figure it out. I checked with our doctors this morning and our daughter's pediatrician and all blood types have been confirmed. Both my husband and I are O+ and our daughter is A+ She is definitely not adopted and unless she was switched at birth, then there is no doubt as to parentage -- should I be concerned? I do recall that when she was born she had mild jaundice which one doc explained was due to blood type incompatibility.
These "anomalies" are quite frequent and they have simple explanations once you know the real genotype. For example, one parent could be heterozygous for two different O alleles. One with a mutation near the beginning of the gene and the other with a mutation near the end of the gene. Any germ cell recombination event between the two alleles could generate an A allele or a B allele depending on the origin of the O allele.
I think it's also possible for one of the parents to actually be AO or BO but the functional allele is expressed at a very low level giving an O-type phenotype. The A or B allele could, by chance, be much more active in the child. There could be epistatic effects such that splicing or transcription is defective in the parent but compensated for by enhanced activity in the child due to unlinked mutations. (These are known as "suppressors" in bacterial genetics.)
There are many other possibilities. They're rare but it's certain that they will show up in some school class somewhere. It's usually not a good idea to investigate the personal genomics of students because of these problems.