Charlie Foxtrot India has hit the nail on the head...
You need to be careful here because the effect of adding full power (particularly on a bumpy grass strip) will full flap can have you airborne again at a speed below your safe Vmca. This can be quite a suprise for a novice pilot.
I have found that the C172 is particularly prone to this type of thing, and the 152 can also be a risk. The correct proceedure in these aircraft is indeed to raise the flaps prior to applying full power, or, at least get them travelling first. Often the effect of all that prop stream over the wing and those big flaps *can* be enough to produce the required lift to leave the ground - particularly if it coincides with a suitable bump in the surface!
If you are concerned that waiting for flap retraction before applying full power will somehow render you short of runway to get safely airborne again, then I suggest stopping and back tracking is the only option open to you. If it is necessary to touch and go then be sure that the strip you are using is an appropriate length!
Don't compromise your safety and that of your student to try and accomodate what may be an unsuitable strip, because if your margins are that tight it rather suggests that full stop landings would be far safer... If the strip is long enough there should be pleanty of time to configure the a/c correctly for the next phase without being in a panic to get the power on.
Nish