Hello there,
In your examples “so” is the correct answer because:
• After “so” we can have an “adjective” or an “adverb”; for example, the exam is so easy(adj), he drives so fast(adv).
• After “such a/an” or “such” we need a noun phrase; for example, it is such a beautiful house (a beautiful house is a noun phrase, meaning it has an article((a)), an adjective ((beautiful)) and a noun ((house)))
Remember “such a/an” is used before singular countable nouns, but “such” is used before plural or uncountable nouns; for example, there are such good teachers on italki that help you learn English so fast! (Teachers = plural); using such furniture is commonly seen in this country (furniture = uncountable noun).