Learn German: Es ist wichtig, dass alle synchron springen. - It is important that everyone jumps synchronously.

Both can mean "everyone," but they take different verb forms. Alle is plural (literally "all [people]"), which is why we use the plural verb springen. Jeder is singular ("each person"), so if you used it here, the verb would change to match: dass jeder synchron springt.

Why is the verb "springen" at the very end of the sentence?

Because dass is a subordinating conjunction. Whenever you start a clause with dass—or related conjunctions like the nur dass you are studying in this lesson—the conjugated verb gets kicked to the very end of that clause.

Could I drop the "dass" and use an infinitive clause with "zu" here?

Not in this exact sentence. Infinitive clauses with zu (like Es ist wichtig, synchron zu springen) don't have their own subject. Because you need to specifically mention who is jumping (alle), you have to use a full dass clause.

In English we say "synchronously." Why doesn't "synchron" need a special adverb ending?

In German, adjectives and adverbs generally share the exact same base form. While English adds "-ly" to turn the adjective "synchronous" into the adverb "synchronously," German simply uses the base word synchron for both.