Ipilot,
Now that we're done having our yucks, I do want to address your origional post in detail, because there are some important issues here.
First of all, don't take the runway until you're ready for takeoff. Don't call ready, until you're ready for takeoff. You indicated that you were waiting for the engine gages to come up in the green. You're not ready for takeoff if your powerplant isn't ready for takeoff. This being the case, you have no business on that runway, period.
Every aircraft in the world will be congregating on that runway at some point. A runway, somewhere. That makes the runway a highly likely place for a collision, and one should minimize one's time on the runway. Be ready to go and very alert when you enter it. Don't spend time sitting, reading checklists, or doing anything else there. When you cross the threshold, you're eyes up, and ready.
I won't criticize your broken english, but I didn't understand what you meant when you indicated you were holding on the runway and another aircraft was cleared onto the runway. If you had been cleared for takeoff, then no aircraft should have been cleared onto the runway. If you were told to position and hold, then there is no issue with you waiting in position, as you're adhering to ATC instruction. Your description of the event does not make sense.
I don't understand why the controller became upset at your delay with your student, if you were instructed to position and hold. This indicates to me that you weren't on a position and hold clearance. Your description seems to indicate that you were cleared for takeoff, and then took the runway and spent time doing pretakeoff checks on the runway. If this is the case, the tower controller had every right to be displeased.
Perhaps you can clarify what actually happened.
It also appears possible that the situation you're describing involved an expedited takeoff. You may be describing a request from the controller to expedite your takeoff in order to accomodate the needs of traffic landing on an adjacent runway. If the aircraft is just touching down and will be rolling onto your runway, this is tantamount to a LAHSO type operation, and I would probably refuse the clearance to take off until I had a good idea of what the other aircraft is doing. Again, there's little detailed information to go on here; all we know is that you accepted a clearance when you weren't ready (and didn't inform the controller), and that another aircraft was involved.
Think about the example this shows the student. Certainly you want to give the student time to accomplish his or her checklist and preparation. Rushing kills. Certainly you want to show the student the proper way to handle this situation. Even in a remidial way, if necessary. Everything discussed here can be avoided by holding short until all your preparations are complete. You shouldn't have to review the checklist on the runway, or write things down, make calls, or do anything else that might divert your attention from the oh-so-important task of takeoff.
Takeoff is the most critical phase of the flight; being on the runway is also a very critical time. You need to strive to show the student the important of this, and the example you set is the single most important thing you can do to reinforce this in the student's mind. I encourage you to review your proceedures and methods, and look at them from the point of view as to what is best for the student. The student is, after all the reason you're there. Good luck!!