Fredericks only moved to Millwall late last week.
Millwall boss Ian Holloway said: "It was a fluke goal. Ryan didn't mean to do that but I'm just delighted that we held on. What I asked for they gave me, and this is the bare minimum.
"I'm getting my schoolteachers in place to help me be the headmaster of this wonderful school. Everybody is going to be encouraged and worked at."
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