Yes, the block stopped the first round. An AR-15 pattern rifle accepts removable box magazines carrying .223 Remington/5.56 x 45 mm NATO ammunition from 10 to 40 rounds without going to drum magazines. (There are, as @GaryM notes, other calibers that can be fired in that platform by changing the upper receiver including the .300 AAC Blackout and 6.8mm Remington SPC that have heavier bullet weights and offer even better penetration.). The typical magazine capacity is 30 rounds (where allowed by law); with that number of rounds you could easily blast a man-sized hole through an unreinforced cinder brick wall or send a large volume of shots into a room regardless of whether a door was secured or not, and of course a moderately skilled person can swap magazines in that rifle in a few seconds.
In general, most objects that people think of as secure cover against gunfire such as wooden tables or doors, sheet metal car bodies, exterior house walls, et cetera are quite permeable to even relatively low energy centerfire rifle rounds like the .223 Remington/5.56x45 mm NATO, and of course an even moderately “high powered” .30 caliber or 7mm rifle round will punch right through a cinder block with plenty of residual kinetic energy. The entire notion of making classrooms into secure fortresses than can withstand deliberate assault by an individual armed with a centerfire rifle (and often with school shooters having knowledge of how to enter and move around the facility) is a kind of survivalist fantasy that doesn’t reflect reality any more than plans to arm teachers or equip children with bulletproof backpacks.
Stranger