I would trust properly trained teachers or administrators over janitors, but some protection is better than none.
Why should a "properly trained teacher or administrator" be any better than a janitor? First of all, most teachers are super-liberals and HATE guns, so I think it is very likely we will have many of them be willing to field some REAL protection for their kids. Also, janitors are probably more practical-minded people to begin with and much more likely to have been using weapons in the past.
Finally, arming teachers in the schools may be better than nothing of course, however, the best option is simply not to attend the schools to begin with. There is no reason all the property tax money and other money going to fund schools could not be given back to the individual in question if he/she does not use the services offered. In that case, we would only mostly have liberals in these schools and then liberal parents could be free to ship their kids to killing-zone day cares, I mean "schools" as before.