Grades

Every average gets a grade. The teacher gives it like this:

• an average of 45 or more: A
• 38 or more: B
• 30 or more: C
• 20 or more: D
• less than 20: F

That's a choice between several options, which is what else if is for. Put it in a function that gets the average and returns the grade. A function can have more than one return, and a condition decides which one runs. The grade is a letter, so the function returns a char: write char before its name, and return letters in single quotes, like 'A'. For example, T-shirt sizes:

#include <stdio.h>

char shirt_size(int height) {
    if (height < 150) {
        return 'S';
    } else if (height < 175) {
        return 'M';
    } else {
        return 'L';
    }
}

int main(void) {
    printf("%c\n", shirt_size(140));
    printf("%c\n", shirt_size(160));
    printf("%c\n", shirt_size(185));
    return 0;
}

S
M
L

The order of the conditions matters. The program tries them from the top and runs only the first one that's true. So each next condition is tried only when none of the earlier ones was true. Someone 140 cm tall also meets height < 175, but the program never gets to that condition. In grade(), start with the highest limit. If the first condition were avg >= 20, it would be true for Finn with 48.3 too, and he'd get a D.

Watch the limits as well: an average of exactly 38 is still a B, so the condition is avg >= 38, not avg > 38.

And watch the quotes: "A" in double quotes is a string, not a char, and the compiler reports an error:

main.c:21:16: error: incompatible pointer to integer conversion returning 'char[2]' from a function with result type 'char' [-Wint-conversion]

On each line, you'll need the average twice: to print it and to work out the grade. So store it in a variable, double avg = average(students[i]);, and average() is called only once.