If A Russian has made soup, he might describe that by saying я сварил суп “I boiled up some soup.” Using that as a starting point, a foreigner might try to use that same verb when making tea. If you do that, the Russians will understand you, but they'll tell you it's better to say я заварил чай. The verbs are actually a bit different in meaning. The pair варить/сварить can mean 'to boil something,' i.e., to actually put something on the stove or over a camp fire or heat source in a smelter. The pair заваривать/заварить can mean 'to pour boiling water over something in order to cook/prepare it.' Russians don't usually make tea by actually boiling the leaves in a pot on the stove; instead they pour boiling water into a tea pot over the tea leaves to make a strong infusion, and then they poor some of the infusion from the teapot into a cup and then add boiling hot water and drink the result.

I was curious to know just how much more common it was to say заварить чай as opposed to сварить чай, so I did a Google search and came up with these results.

Сварить

Заварить

As you can see the заварить versions outnumber the сварить versions by a factor of 15. When there is that big a difference, you can teach your beginning students that the заварить version is right and the сварить version is wrong.