Praised for his outstanding math abilities at such a young age, Isaac is probably the greatest kid you'll ever meet! However, he might hit you with a ruler, and that's pretty messed up.
When the player first enters the game, they are of course met by Baldi, who waves right at them and welcomes them to his schoolhouse. Behind Baldi are his twin sons Jack and Isaac, who hide from the player, attempting to trick him/her into thinking that they're very shy and sweet.
After the player collects the first two notebooks (and after they answer the third question incorrectly on the second notebook), there are multiple ruler slapping noises coming from the three corridors.
Isaac is the most difficult one to get around, he will usually move in front of the player, making it very difficult for the player to get around him and move on. Every time the player answers a question incorrectly, the ruler slaps will become louder and faster.
Once the player collects all seven notebooks, and after Baldi states: "Congratulations! You found all seven notebooks! Now, all you need to do is GET OUT WHILE YOU STILL CAAAAAN! Heh heh heh!", both Jack and Isaac will move faster than Baldi, making it difficult for the player to escape.
(NOTE: Instead of giving the player a jumpscare, either Jack or Isaac will teleport the player to the Principal's office, where the player will be met by the Principal, who gives him/her a 30 second detention.)