Ahhh, individuual aircraft instruments...I agree, if a club/school can't fix them for their own reasons and there are lots of u/s stickers in the cockpit, that is indeed sloppy....
However, in my experience (very limited I admit (!)) even a u/s instrument or tyre will make the entire aircraft u/s. If a club/school took a student flying without for instance an altimeter working...or the static pressure vent blocked, I would start to worry..... It gets the student into sloppy habits in their check walk round...the old adage I work by is "A superior pilot uses their superior knowledge so they don't have to use their superior skills..." In this instance, using ones knowledge (however limited, again I am no expert!) to tell that an aircraft is not airworthy, and therefore does not fly....