Takai is an adjective.
形容詞は名詞を修飾することができます。
Adjectives can modify nouns.
The adjective 'takai' (high) modifies the noun 'yama' (mountain).
If you turn 'gakusei' (student) into an adjective, you get 'gakuseiteki'.
stem of an adjective; adjectival stem
adjectival