Jaime mentioned how the hologram was set up - they put up projectors and such all around the school, no mention of a robot. Which is a rather impractical suggestion in this case - a robot a) wouldn't need the projectors and speakers in the school (it would have them built into itself), and b) would be rather a larger bit of hardware to inconspicuously store in a building he didn't have full access to. (And which probably didn't have many unused spaces to begin with.)