The council is in talks with the Department for Education and Skills to find an expert to lead the inquiry. It will look into whether the school made adequate preparations for the trip, why Bunmi's disappearance was not noted earlier and whether the lake at Cany-Barville was a safe place for swimming.
A council spokesman denied reports the school had not been fully covered by insurers for the risk of pupils swimming in the lake.
It was the first time Hillmead primary school had taken children abroad on a trip. Six members of staff were supervising 41 children.