Outdated client! Please use 17w06a

I have updated all of the computers at our school that run Minecraft. However, two of them (one a mac and one a windows machine) keep giving the error message "Outdated client! Please use 17w06a" even after being updated...then repairing....then uninstalling the application and reinstalling it. They are giving this message when I try and join them to the local lan. Help