Pam Mathers had just reached the 26 mile mark when two bombs exploded.
Students gave the principal high fives as she completed the marathon inside Hamilton Elementary School near Detroit.
"We all felt bad that she couldn't finish the 26.2 miles," fourth grader Ryan Smalley said. "So, we decided that we would help her finish these last point two."
"It's incredible," Mathers said. "The students just, you know, they make everything, win or lose, they make everything worth it."
Mathers crossed an improvised finish line inside the school library. She also received a medal that she was supposed to get after completing the marathon on Monday.