If you ask Valerie Gaynor which is tougher, a shark or a high school teacher, you can bet which one she'll choose.
"Oh definitely a high school teacher," she said, laughing.
This high school marine biology teacher from Jenson Beach, Fla., swims with several different species of sharks in the wild -- including great whites and whale sharks.
"Last year I did the sharks that people say are the scariest sharks, this one, I wanted to do the largest fish," Gaynor said. "They're so gentle. They're the gentle giants of the sea. They're incredible."
Gaynor and her husband J.D. allowed ABC News cameras to take a boat trip with them 20 miles off the coast near Cancun, Mexico, to what might be the world's busiest feeding ground for whale sharks.
These fish measure up to 41.5 feet long -- that still a few feet longer than your average yellow school bus -- and weigh roughly 47,000 pounds. They are enormous but harmless.
"I don't even thing that they notice that we're there other than the fact that there's someone in an orange life vest chasing them," Gaynor said.
Divers can get within inches of these swimming dinosaurs and feel their movements through the water. For Gaynor, who has been diving for almost four years, its more than a hobby, it's a lesson plan -- and an educational philosophy.
"I want to bring an adventure to my kids ... because it brings the excitement back into the classroom," she said. "Right now, we're so geared on testing, testing, testing, if I can teach them science through something amazing and fun, then they're going to listen to me, they're going to pay attention."
Gaynor's husband films all of their underwater encounters with sharks, which they pay for out-of-pocket, and she brings the videos back to the classroom.
"My grandmother was a teacher in a one-room schoolhouse, and that was in northern Illinois and she showed me everything I know," she said. "She showed me if you show the kids you care, you'll have them forever."
In a way, sharks give Gaynor "street cred" with her students. She is fearless when it comes to swimming with them and is committed to their conservation.
"My dream would be that we could conserve them so they would be here for generations," she said. "Right now, because of what we're doing with the shark fin trade industry, we're depleting them out, millions of sharks every year."
Gaynor said she has never had a run-in or a close call with a shark, they just don't want to eat humans, she says.
"I see it smiling and it's happy and it's just living its life like all of us," she said.
The thrill she gets from bringing an underwater world to her students, she also gives to them. It's no surprise that Gaynor was voted best teacher of her school's county last year.
"I love the spark," Gaynor said. "When the spark goes off in the kids' eyes and they say, 'Wow! I can't believe you did that! Are you kidding me Ms. Gaynor? This is incredible!'"
She is using sharks as shark bait to reel in her students' attention to science, and it's working.