In addition to what CPallini has said, I'd change it a little to use a function to return the grade, and print that:
public static String getGrade(int score) {
if(score >= 80) {
return "Excellent";
}
else if (score >= 50 ) {
return "Good";
}
return "Low");
}
And then use that to print the result:
System.out.println("Grade: " + getGrade(studentScore));
It's then a lot easier to move on the the second part of your task and repeat the scoring for the remaining 134 students, knowing that the "grading" part of the task is done and working.