I would recommend a conventional telecare alarm system which can be used either with a fixed telephone line or with an integrated GSM output so that it can be portable and used virtually anywhere. The teaching assistant would have a small panic pendant while the dispersed alarm unit could be programmed to ring the mobile phones of relevant staff using a silent dialling mode as well as the central school switchboard. The staff member' phones would be on vibrate mode for these calls so that nobody in the class would know what has happened. 
This is the most robust approach as it uses commercially available and low cost communications infrastructures. The range is unlimited as far as the responders are concerned (provided there is mobile coverage) while the range from pendant to dispersed alarm unit can be as high as 100 metres if the VHF rather than UHF option is selected.  The systems can be provided by Tynetec and offer a range of pendant colour options so that multiple users can use the same dispersed alarm without confusion. Commercial systems are also available from Tunstall and Chubb, though the range of wireless transmission tends to be less in buildings.
Incidentally, such telecare systems could also be used with a wrist worn fall detector as a man-down detector and with the panic bars used in some facilities where clients may exhibit challenging behaviour.

Cheers,

Kevin

Dr Kevin Doughty
Co-Director
Centre for Usable Home Technologies
University of York and Newcastle University