i am sorry, but what! foot over bridges can be made in months, if not weeks. they are not that complex (just lots of metal rods on either side of rod, and middle truss are manufactured externally, just shipped and bolted). there is a possibilty, that we may be having a language gap (though we both speak english, we may use different words), so i would request you to please check if these are sufficient for school's needs - https://en.wikipedia.org/wiki/Footbridge
the wiki page disadvantages list cost, but that is considering we take care of accesssibilty, and install elevators or escalators. but simpler ones can be built under $10,000, if you go all metal and glass decks, that would double or triple.
where i live, these are fairly common, and used on almost all major roads. We are densely populated, so not everyone can have cars even if they have financial means, because all roads would be choked.
additional note - wiki's images on fist half of page are mmostly weak wooden or natural aided ones, but lower down there are modern designs. take for example the one over train tracks (though less wide than your highways i am guessing, but take it as an example). They are simple enough, and you would only have to get highway's permissions mostly to ensure that height is enough (usually highways have a height limit, something like 20-30 feet (2 or 3 storey building)). now most people are not willing to go 2-3 floors, so escalators are common. so "small tunnels" are more convinient. just imagine these bridges, but under ground and upside down. now assuming the beneath of road is reinforced, you just need to dig like 10feet below, and you are good to go. they are more expensive when built after building roads, but still can be built under 100,000