Should it not be "Magister vester peritus est"?
I haven't studied latin in a few years, but Im fairly certain it is at least correct to use the verb last in this case.
I'll report this as well.
When is "tui" used
"tui" is for the singular "you" when we're talking about more than one masculine thing.
"vester" is for the plural "you" when we're talking about exactly one masculine thing.
Wouldn't it then be 'magister tuus' in this case? And if it's multiple, then 'magistri tui'?