Police said Cassell was riding southbound along Old Georgetown Road on the sidewalk when he fell into the roadway and was struck by a Volkswagen Atlas. The driver, who was not hurt, stopped and stayed at the scene, according to police.
Cassell dressed up as the bulldog mascot for Winston Churchill High School in Potomac.
“It is with a heavy heart that I share news of the passing of our very own Bulldog, Jake Cassell,” school Principal Brandice Heckert said in a Twitter message.
She said he would have been a junior in the fall and had a “kind heart and a contagious smile. He will be greatly missed.”
Officer Rick Goodale, a Montgomery police spokesman, said investigators are trying to determine why Cassell, who was wearing a helmet at the time, fell off the bike.
Goodale said the location is not known as a spot for a high number of crashes. The speed limit is 35 mph.
Heckert said the school received a new bulldog costume Wednesday after ordering a replacement because the existing costume had become worn. She said she was looking forward to giving it to Cassell in the fall.
“He was so excited about it,” Heckert said, “and he deserved it.”
The new costume was selected for Cassell, with a head that portrayed a smiling bulldog, rather than the more “intimidating” older costume, Heckert said.
“We got one that matched Jake’s personality,” she said. “He always had the biggest smile on his face no matter what. He was just a happy kid.”
Heckert said Cassell had only served a year as the school’s mascot, but was proud to have the role. She said he went to more than 50 sporting and community events during the school year while wearing the costume.
Heckert said Cassell enjoyed being the mascot because it allowed him to meet different people in the school and community.
Cassell was also known for participating in community service projects, including one with his family in which he helped collect and donate backpacks sent to Honduras. He had recently returned from a mission trip.
In an email, Bruce Adelson, who served as the president of the school’s Parent Teacher Student Association for the past two years, said Cassell dressing up as the school’s mascot was a symbol of the “true dynamism and vibe” at the school.
“He will be sorely missed,” Adelson said. “We grieve for Jake with his family and friends.”