Hi F. -
If she has only recieved one C then yes, restricted to house until the next report card (usually 5 wks) is a little excessive.
You can use this as a learning tool. By this I mean, you can go talk with her explain that you reacted when you were upset and after you had time to think about the situation, believe that the restricted time was a little extreme due to the situation. Move it back to maybe 2 wks.
This is letting her know that even adults make mistakes and facing the mistake and apologizing is not only ok, but the better thing to do.
Find the route of the C is the key is there problems at school or not enough sleep or to much tv, computer, cell phone etc, remove that obsticle for the 2 wks so she can focus on grades. Put the stipulation on that if the grade is not raised by the next report card, you will readdress the issue.
My daughter (11) is usually in 90's also, but has been bringing home some low 80's and some 70's. You would be amazed at just one bad grade having the effect to drop you that much. I have had talks w/ teachers to find out why and maybe they had one bad day, which was an important day, that is all it takes. In gym she missed turning in a report on tumbling (dont ask) and dropped from 100 to 83.
Im sure your daughter is upset about the grade, reasure her its ok, but you really would like her to try her best at all times and maybe have a tutor session after school 1 day a week for a few weeks. My son struggles w/ his Latin (dont ask me why he chose Latin!) and our school has a different subject tutor every day of the weeks so kids can stay and get extra help with difficult classes. This has brought his grade from 60's to 80's in just 5 wks.
Best of Luck! Remember its ok for parents to be wrong, how we handle it will teach our children.