While I probably wouldn't use "dismissed" in this case, I don't think it's rude or for the military.
Teachers dismiss students from class at the end of the period, or a school principal might dismiss a student from her office after talking to him. It just means, 'you can leave', as in 'I'm in charge and I give you permission to leave.'
I like "That will be all."
"That will be all, thanks" or a variation is the best choice. A teacher might tell a group of students that they are dismissed, yes, but that's a narrow use of the word. Telling an individual that s/he is dismissed, other than in a military context, would be rude.