You are placed in a group with other learners and ranked by the XP you earn that week. In a full group, the top 10 move up a tier and the bottom 5 move down. There are seven tiers: bronze, silver, gold, sapphire, ruby, emerald and diamond. Everything resets when the week turns over.
Small groups work differently
A group needs 15 members to be treated as full. Below that, the top third are promoted and nobody is demoted at all.
The reason is that the higher tiers are thinly populated, and applying a bottom-five rule to a group of eight would demote most of it every week. Reaching diamond would then mean bouncing between diamond and emerald indefinitely, which measures nothing except how many other people happened to be there.
Zero XP cannot promote you
Promotion requires having actually earned something that week. If you score nothing, you are not promoted, no matter where the ranking puts you.
This matters more than it sounds in a quiet group. If every member sits at zero, ranking alone would still produce a top ten, and those accounts would be promoted for doing nothing — then promoted again the following week from the tier above. Inactive accounts would drift steadily upward and fill the top tiers with people who were not there.
Demotion has no equivalent protection, and that is deliberate. A week where you earned nothing is exactly the week the bottom five should include you.
Ties
Two people on the same weekly XP are separated by a stable tie-break rather than by who got there first, so the ordering does not shuffle between one view of the table and the next.
What actually earns league XP
The same XP as everything else — there is no separate league currency. Correct answers in any mode count, at that mode's rate, and a good combo in timed Match can double a round's contribution.
Which means the fastest way up a league table is Scramble, at 3 XP a word, or timed Match played well. Whether that is also the best way to learn is a separate question, and the honest answer is that the league is measuring effort, not retention.