Bunmi's body was found three days after she vanished
The 11 year old drowned in a lake near Dieppe while on a trip with her school, Hill Mead Primary in Brixton, south London.
The British embassy in Paris said that the magistrate, appointed on Thursday, would sift through all the evidence with a view to a possible prosecution for "involuntary homicide".
Bunmi was with fellow pupils and teachers at the swimming lake when she disappeared on 2 July.
Her body was recovered from the water three days later. A post-mortem examination showed that she had drowned.
Police took statements from pupils, teachers, and staff at the swimming lake.
Under a cloud
An embassy spokesman said those would form the basis of the magistrate's inquiry - which might take "weeks, running into months".
At this stage of the process no blame attaches to any individuals and it is unlikely the teachers will have to return to France for questioning.
But undoubtedly a cloud will hang over them, the spokesman agreed.
Last year another English teacher, Mark Duckworth, was given a six-month suspended prison sentence by a French court after being convicted of the manslaughter of a 13-year-old pupil, who drowned while on a school trip to northern France.
An appeal has been lodged on his behalf.
Different approaches
A lawyer familiar with that case said that the French guidelines on school trips were broadly similar to the English ones, but there was an important cultural difference in the two legal systems.
"We have a greater tendency to draw things like this out of the criminal courts and into commissions and inquiries," he said.
If there were to be a criminal charge it would probably be criminal negligence.
In France, it was a criminal matter. The likely charge - manslaughter.
"In England, inquiry reports can be critical of those involved but the main thrust is to learn lessons.
"But in France and many European systems, it is a semi-punitive approach aimed at deterrence," he said.
"A label attaches to all those involved right from the outset.
"There is a danger of witch hunts, with people being blamed when it later turns out they weren't at fault."