M.,
It is probable that if she has a speech delay, that she was delayed from the get go, but it is also possible that she met early milestones and now has a delay.
The school is a great resource, however, it is not going to be enough. Schools are required to identify children with needs and then make her functional in the classroom setting. I don't know about you, but I have never met a Mom who said "I want my child to be functional!" You want her to live up to her potential, and the school is not required to do that.
Continue with the school evaluation, but also call a speech therapist and make an appointment for a private evaluation and start therapy asap. Even if you have to pay, more intervention as early as possible is what she needs. If I am picking up on the tone of your post right, you are kicking yourself that you did not pick up on this any sooner, but the people you pay (your pediatrician and your educators) should have warned you sooner, especially her pediatrician. That is all too common unfortunately, but don't beat yourself up, just get her what she needs right now.
Dealing with the school can be difficult. There is a web site www.wrightslaw.com that can help you understand. There is no set timeline for speech therapy, some kids need 120 days, some need therapy for years. You will get a prognosis from your private therapist. Please get the private evaluation, you will not be sorry you did. You never, never want to know less than the school about your child, and you want to own the evaluation that holds her diagnosis.
Good luck, again, don't beat yourself up.
M.