OP, I know it was probably a nerve-racking situation for you, and thank goodness nothing bad happened, but try to look at this way: Your child got on the wrong bus. You didn't realize this until she didn't get off at the bus stop, so that could be 20-30 minutes (or more) of her being in the wrong place. When you called the school, she was immediately located and then brought back to the school, and you were kept updated of the situation.
Mistakes are going to happen--the real test is watching how those mistakes are handled. Sounds like the school has a solid plan in place and it worked exactly as it was supposed to. This may not be the last time your child takes the wrong bus--there could be a substitute bus or teacher, she might get mixed up, or there might be another situation similar to this day, when she was supposed to do something other than her routine. You can now be assured that if it does happen again, she will still be safe and it will get resolved quickly.