Here's why:
* Short vowels are usually followed by a consonant. In "student", the vowel "u" is followed by the vowel "e" (which is silent in this word).
* Long vowels are usually followed by a vowel or a consonant with a silent "e" at the end of the word. The "u" in "student" sounds like a long "u".
Therefore, "student" has a long vowel sound.