Q. Do open compounds like “face mask,” “cell phone,” “sea level,” “high school,” and “life science” (all identified as nouns by Merriam-Webster) have to be hyphenated before a noun?
A. If a compound is listed as an open (unhyphenated) noun in Merriam-Webster, and the text seems clear without a hyphen, then you can leave the hyphen out even when the noun is used attributively (and even if the rules for hyphenation in CMOS 7.89 seem to suggest otherwise):
The face mask rules did not apply to cell phone users living in areas subject to sea level rise, regardless of the number of life science courses being taught to area high school students.
That seems clear enough without any hyphens. But if you do hyphenate any one of these compounds, you should hyphenate it everywhere else that it’s been used attributively. A consistency checker like PerfectIt can make this job a lot easier. See “Hyphenation in Context: The Chicago Manual of Style for PerfectIt,” at CMOS Shop Talk.