UP IN ARMS means that there is strong opposition or anger against something.

For Example:

“We heard the news over the radio at lunch break and I tell you, we were really up in arms about it.” (Meaning they strongly disagreed with the news on the radio).

When their children have to go to a run-down, under-funded State school, perhaps politicians will better understand why most of the country is up in arms about the education system. (Meaning the country is generally very dissatisfied with national education).