Harman said police have launched a search for a local man, 51-year-old Terry Glover, in connection with the attack.
Officers are hunting for a silver Ford KA that left the scene following the horror hit-and-run.
Earlier, the Essex Live media outlet reported, citing witnesses that "multiple children" have been hit by a vehicle outside Debden Park High School in Loughton.
Essex Police confirmed they're on the scene, on Twitter it mentioned: 'We're at present on the scene of a severe collision on Willingale Highway, #Loughton.
"We are investigating whether or not this incident may have been connected with another incident nearby".
"Man arrested after fatal crash outside school - we have arrested one male for murder following today's Loughton incident - thanks for all your help and support with this investigation", tweeted Essex Police Serious Collision Investigation Unit Inspector Rob Brettell.
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Essex Police said they were treated at the scene or in hospital, and their injuries were not believed to be life-changing or life-threatening.
Call 101 and quote incident 726 of Monday December 2 with information.
A serious crash reportedly took place near a secondary school on Willingale Road at around 3:30 p.m. local time, the Essex Live media outlet reported.
Debden Park is a mixed academy school for pupils aged 11 to 18, which specialises in the performing arts and received a rating of "outstanding" in its most recent Ofsted inspection, according to its prospectus. As a school we continue to work with the police supporting the ongoing investigation.
Chief Supt Harman said: "At Essex police and on behalf of all of all those called to the school our thoughts are with those, his families his friends and all those that were injured or affected by what has happened this afternoon".
The area remains cordoned off amid a large emergency services presence.