During a touch and go (C150) should I first raise flaps or set full throttle? I am now finishing my PPL (on a C150 in Czech Republic) and today had quite a close call when doing a solo touch-and-go on a short runway. We train on grass (and therefore rarely completely level) runways and take off with flaps 10 (landing with 20 or 30). I stayed in ground effect a couple of seconds before touchdown and then, according to my instructor's (hmm) instructions, first raised the flaps to 10 and then applied full throttle (and carb heat full cold).

I saw trees approaching and cleared them by an uncomfortably small margin (which made my further way back on which I managed to lose track a bit seem like a joke I wouldn't care about).

The explanation I heard from other flying guys is that touch-and-go should be an imitation of landing followed by a take-off, and one should therefore retract flaps (to 10) and then apply full power. And if you are not sure of runway distance left, you should make a full stop and take off anew.

Does this actually hold true? Imitation and practicing is all good and well, but assessing the runway distance left (and more so at an unfamiliar unlevel grass field I was landing at) in one second you might have may prove too risky and may lead to quite different consequences I was lucky to have avoided.

The instructor actually confirmed they were planning to change their route for the long nav flight because this field proved too challenging.

The question is, however, generic: first raise flaps or set full throttle?
Flaps in a C150 (and pretty much any type) take a few seconds to move. Throttle change is more-or-less instantaneous. If you were to throttle up first, and then change the flap position there is a good chance that you are lifting off while the position of your flaps are changing. This is not a good idea, changing the amount of lift generated by the wings in this crucial stage of flight can lead to a take off stall. You could (and probably will) be too low to recover.

To minimise the chance of this happening, I would say that on touchdown, one of the first things I would do is to set flaps for takeoff ( giving the actuators a second or two to change flap position) before immediately setting take off throttle.

So, yes, your instructor was right. They usually are. In your specific case, I suspect that as soon as you floated too far down the runway, you should have initiated a go-around before touching down. You did not leave yourself enough runway to safely perform a touch-and-go. You dont assess take off distance available once you touch down, you asses it while still flying - you had ~10 seconds to make that call (approach, float, oh shit, go around!)