As already mentioned, winds can be highly variable depending on the time of day - generally calmer early in the day before the air starts warming. Also they can be highly variable based on altitude and the terrain - whichever source you’re using for the weather forecast could be showing projected wind speeds that’ll end up being very different from the conditions at your launch site.
One good online resource for checking wind speeds is
https://www.windy.com/ which you can adjust by altitude - with Estes Vikings on small motors the wind speed at 500-1000 feet is pretty much irrelevant since you’re only going to hit maybe 200 feet or so on a 1/2A and maybe 400-ish on an A10 so looking at different wind speeds at different altitudes could be useful. And checking the projected speeds over time could be handy for planning your launch windows - I like Accuweather for the hourly forecast but all the usual weather sites/apps have hourly forecasts so use whichever you prefer.
If the winds go completely sideways on you here’s a suggestion that might sound a bit extreme but it’s worth thinking about - switch gears completely and have your students build one of Art Applewhite’s folded paper “saucer-style” rockets (
http://www.artapplewhite.com/free/index.html) - you can’t get much cheaper than a package of lightweight card stock and some glue sticks, they build up fast and easy and fly surprisingly well. I’ve used the Cubit design for build and fly sessions successfully several times. Not as mainstream or “rockety” as your Vikings but flying anything is better than flying nothing
Good luck - hopefully you have some clear calm weather and things go well!