That must be so frustrating! What does your pediatrician say? Maybe a checkup is in order after the holidays, see if the pediatrician has any ideas?
My 12yo son has been sick on and off this whole school year with one cold/bug after another. It has been truly excessive for his age IMHO. Nothing that has required antibiotics thus far but have made an appt with our ped after Holiday break to get an exam and some bloodwork or whatever the ped thinks we need. It is probably nothing, but he is my 3rd kid and it seems odd enough to me -at this age- to be a bit worrisome.