Mr Simpson told police on Sunday he realised he had left the legally registered pistol in the toilet at the Deerfield Beach Pier after hearing a single gunshot.
After running back to the bathroom, the science teacher encountered a homeless man holding the gun, who, he added, appeared drunk.
Police arrested Mr Simpson and Joseph Spataro, 69, at the scene.
Mr Simpson was charged with failing to safely store a firearm, and was released after paying a $250 (£175) cash bond.
Mr Spataro, who told police he fired the Glock 9mm handgun to see if it was loaded, was charged with firing a weapon while intoxicated and trespassing.-BBC
A teacher at Marjory Stoneman Douglas High who said he’d be willing to arm himself at school to protect students has been accused of leaving his loaded handgun in a public restroom, where it was picked up and fired by a drunk homeless man. https://t.co/MHHopF1IZq
— Kyle Griffin (@kylegriffin1) April 12, 2018
Why? Instead of legally killing someone how about sending to Federal Prison. This guy is not comparable to the Parkland Shooter, Nikolas Cruz who killed 17 people. Most of them children, And few were adults. And also caused a national security threat towards education facilities
— ????♂️Will Bell??♂️?? (@MilitaryGamer98) April 12, 2018
Parkland teacher arrested after leaving loaded gun in public restroom https://t.co/dWRZp2kZDu
— 321 Training (@321Training) April 12, 2018
Let's arm the teachers!
— El Hopaness Romtic (@rayhatfield) April 12, 2018
Retweeted Washington Post (@washingtonpost):
"We’re kids, but we’re also journalists.”
How Parkland student journalists covered the shooting they survived and friends they lost https://t.co/uuPHNIbNbO
— Phyllis A. Gilchrist (@PAGilchrist) April 12, 2018
— Debbie McCall (@1DebbieMcCall) April 12, 2018
HOW'S that arming the teachers thing working out??? BBC News – Parkland teacher arrested after leaving gun in public lavatory https://t.co/xyaDBbzoZj
— Stop the Lies (@stopthelies1) April 12, 2018
— Carol Forden (@CarolForden) April 12, 2018
'This would mean more guns': Broward County won't arm school employees after Parkland shooting https://t.co/fAdS9eKUux
— thehawaiiguy (@thehawaiiguy) April 12, 2018
Parkland Gunman Nikolas Cruz Offers to Donate Large Inheritance to His Victims Families https://t.co/MzzD5nYjQE
— In Spite of Trump ? (@InSpiteOfTrump) April 12, 2018
— WESH 2 News (@WESH) April 12, 2018
Unbelievable! ACTUAL Parkland shooting victim totally ignored by school district while the #BRATivists & administration are all over MSM! Disgusting! Take care of the REAL VICTIMS BROWARD! This is unacceptable!
Great job Lisa Olsen, I'm glad @POTUS had time for your son. ♡♡ https://t.co/tFhYLsuPRo
— TaylorMade (@RedGirlInBlueCa) April 12, 2018
— JLEE (@bunnydear) April 12, 2018