When the total bill is $25 share of each student becomes 8.33 and not 9

The minimum is three.

Start three planes from the island (yours and two others). After going 1/8 around, one plane refuels both yours and the other plane with 1/4 of a tank each, leaving it with 1/4 of a tank which is precisely enough to take it home.

Two planes continue with a full tank again.

Reaching 1/4 around the other plane transfers 1/4 tank of fuel to yours, leaving it with 1/2 tank, to take it precisely home again.

Your plane is now full at 1/4 and can go to 3/4 with that fuel.

The first returning plane refuels and start off the other way meeting your plane at its 3/4 position, where your plane is empty and the meeting one is half empty. Sharing their fuel they have both 1/4 tank left which can take them to the 7/8 position.

The remaining plane has arrived there in the meanwhile with 3/4 tank giving 1/4 to the other now empty planes.

All three planes have now 1/4 and can return home.

What a trip!