Many U.S. publishing houses follow the Chicago Manual of Style's guideline on when to hyphenate compound modifiers. here is the recommendation in the sixteenth edition of this style guide:
7.81 Compound modifiers before or after a noun. When compound modifiers (also called phrasal adjectives) such as open-mouthed or full-length precede a noun, hyphenation usually lends clarity. With the exception of proper nouns (such as United States) and compounds formed by an adverb ending in -ly plus an adjective (see 7.82), it is never incorrect to hyphenate adjectival compounds before a noun. When such compounds follow the noun they modify, hyphenation is usually unnecessary, even for adjectival compounds that are hyphenated in Webster's (such as well-read or ill-humored).
In short, Chicago considers
Plan and teach well-structured lessons
to be clearer on first reading than
Plan and teach well structured lessons
(since readers might entertain the possibility that "well" attaches to "plan and teach" rather than to "structured"). On the other hand, no such shadow of ambiguity attaches to
Plan and teach lessons that are well structured
so no hyphen would be necessary (or, arguably, appropriate) in that case.
As others have noted in comments and answers, British style on hyphenation is less aggressively pro-hyphen than U.S. style, but I don't think that anyone could reasonably argue that including the hyphen in the particular case that the OP asks about is utterly superfluous or simply wrong.