Bonnie, as you develop your voice, it is always part of the exercises to go higher and higher (or lower) in vocal exercises and also in adding songs that are more demanding. Each new challenge to your voice builds on the last challenge that you have met. You're building a structure block by block.

Here are some tapes that are available on the internet. I don't know anything about the these voice coaches, but for those without a personal teacher, they may be useful.
click here

-alice