There was a confirmed case of pertussis (whooping cough) at my son's school...?

I really don't understand this. I thought that all school aged children had to have a series of vaccinations which include dtp...I know my kids did. So how does something like this happen? Can someone still get pertussis even if they've had a vaccination for it?

Unfortunately the vaccine against pertussis only lasts for a few years. Most adolescents (and adults) who were given vaccinations as children (the last booster is at age 5 or 6)loose their immunity (immunitey "wanes")) by the time they are young adolescnets and adults. Since many cases of pertussis ("whooping cough") are not identified as such in this age group ( sometimes they are just called 'colds")- an older child can go around for many days infecting many suseptible contacts before the individual is actually treated.
Fortunately- there is now a new vaccination- the TdaP which the old "tetanus booster" which now can be given to adolescents with protection against Pertussis, and unless your child was a very close contact of this classmate ( had close friendly relationships over a period of days or weeks), he probably was not infected from this individual.
however, if your child is having prolong "cold symptoms" (recurrent coughing spells, difficulty sleeping from the cough, etc.) he can be checked and easily treated for pertussis. I hope this explanation helps

Some people are against vaccination and will refuse it for their kids. It's kind of silly, but it does happen.

Another explanation is that the kid has severe allergy to eggs... Since vaccines are traditionally produced in eggs, it is very dangerous, and can be lethal for people who are allergic to eggs to get a vaccine.

Also, most vaccines are not 100% effective. Immune response vary from person to person. I do not know the figures for this particular vaccine, I do think it must be highly effective since it his so widely given, but an odd case-in-a-million of infection can still happen.

I have never had this vaccination due my mother's concerns which I completely agree with. I don't know where you live, but here in Ireland, if you don't want your kids to have it, they don't have to

Yes some people might still get it, even if your son is vaccinated she should recieve and antibiotic (erythromycin) course if he was in close contact with the other kid.