skylark vs lark about

skylark is a noun but lark about is not a noun.

Word	Noun	Adjective	Verb	Adverb
skylark	Yes	No	Yes	No
lark about	No	No	Yes	No

As verbs, skylark and lark about are synonyms defined as:

Other synonyms of skylark include cavort, disport, frisk, frolic, gambol, lark, rollick, romp, run around, sport.

skylark (noun)	lark about (noun)
brown-speckled European lark noted for singing while hovering at a great height

skylark (verb)	lark about (verb)
play boisterously	play boisterously