A degree or a certification is not justification for a raise or to correct some perception of a discrepancy in pay parity. You get paid based on the value of the service you provide.
Yesterday you were a net admin with five years experience. Today you're a net admin with five years experience. The only difference between yesterday and today is that today you have a degree. Are you adding more value to your company because of that fact alone?
I'm going to guess the counterparts you're talking about that make $15k more than you are also net admins with five years experience. (Let's assume that for the sake of making this a little easier.) Are your duties the same as theirs? Are you responsible for the same breadth of service as them? Is your network topology as complex as theirs? Is your network as continuously available as theirs?
If the answers to all those questions are yes, then you've got a case for a raise. You can point to other people doing the same job as you, at the same level as you, in the same kind of company as you, providing the same value as you, but making more money than you.
However, don't bother going into management and saying people at your level with your education make more money than you and therefore on the basis of that alone you deserve a raise. You've been getting an excuse up until now, and they'll just come up with a new one.
One other thought...don't worry so much about being tactful. When asking for a raise or a promotion it's more important to be assertive than tactful. Tactful people are the ones who hear "no" a lot. Be tentative and you'll be dismissed. Be self-confident and you'll be respected. Your choice.
