Several issues

• return in the loop will stop the loop. It is not needed in a loop It is needed in a map or filter and not allowed outside a function

• You need to access the item with [i]

• You have a mix of letters and digits. You need to treat them separately

• "B..." is > "A..." so we need to test the actual string

Here I test the letters with isNaN (is not a number)

It passes all letters > C- and all numbers >= 3

const allStudents = [ 'A','A-','B-',1,4,5,2 ];

const getPassingStudents = () => allStudents
  .filter(student => {
    if (isNaN(student)) 
      return "ABC".includes(student.slice(0,1)) && student != "C-";
    return student > 3;
});    
    

console.log(
  getPassingStudents()
)

let allStudents = [
    'A',
    'B-',
    1,
    4,
    5,
    2
  ]
  
  let studentsWhoPass = [];

  for (let i = 0; i < allStudents.length; i++) {
    if (allStudents[i] == 'A' || allStudents[i] == 'B' || allStudents[i] == 'C' || allStudents[i] == 'A-' || allStudents[i] == 'B-' || allStudents[i] > 3) {
        studentsWhoPass.push(allStudents[i]);
    }
  }
console.log(studentsWhoPass);

NOTE:-

• return statement can only be there in any function any outside the function, you can just update and push the value into the array
• You were trying to push whole array put here we'll just push the element in the array e.g. studentsWhoPass.push(allStudents[i]);
• In the if statement, you were checking whether the whole array for a certain condition but you will have to check for the element same like the above example
• for checking for the character, we could check using the ASCII code but it could become confusing, to make it simple we are checking for individual letters
• Also, instead of &&(and) operator we are using ||(or) operator so that every condition for every element need not be true but atleast one condition should be true

I think this will solve your problem, Thank you!

I'd suggest creating a map of each grade along with its equivalent score, then it's easy to filter students with an equivalent score of 3 or above using Array.filter()

let allStudents = [ 'A', 'B-', 1, 4, 5, 2, 'C-' ]
  
// One can customize as necessary..
let gradeMap = { 'A': 7, 'A-': 6, 'B': 5, 'B-': 4, 'C': 3, 'C-': 2};

const passingScore = 3;

function studentPassed(input) {
    return (gradeMap[input] || input) >= passingScore;
}

let studentsWhoPass = allStudents.filter(studentPassed)
console.log('Passing students:', studentsWhoPass)
    

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