Seeking suggestion on ways to get temporary mobile Internet access

[mikets]

One of our issues during build season is to get temporary mobile Internet access for checking updates, searching for datasheets etc in the field. We usually need Internet connectivity in two places: our school, for which although the school district has Internet connectivity, students are not allowed to access it. At the Joint Practice Field that several schools in our region built in some sort of warehouse where there is no Internet. In the past, if we are luck, some mentors may have a 3G/4G link on their phone that we may be able to tap into. We also consider a temporary subscription to 3G/4G/WiMax type of connectivity. But we are not able to find a service provider that will not charge you an arm and a leg for it. They usually involve activation fee, a 1 or 2 year contract etc. We just need it for the season (about 3 months). What is the most cost effective way to get temporary mobile Internet?