Hi Simon if you are using the MakerBot Mini then the parameters can be changed. You have to create a profile then you can alter things such as the speed the machine lays down the raft and the initial layers of the print, this will hep it stick to the print bed better. The software is set on medium settings to give a pre defined print quality but as the Makerbot brand has stemmed from an open source back ground you can change nearly all of the settings in the Profile section of the software.
As for the print speed, yes you are correct you aren't going to get 15 modes per 1 hour lesson, but if you view the Printer as a similar tool to be use alongside a CNC machine, milling machine or laser cutter but for rapid prototyping and design, production and evaluation then 3D Printing does offer excellent speeds and output. If using in a primary school the main benefit is engaging the pupils in the CAD aspect as often his is the first experience they have of this, designing Mincraft style Zombies or buildings is ideal.