The fixed code is shown below. input() function already returns string that's why you don't have to convert string again. Also the syntax in if-else scope is wrong.
grade = input("What year of high school are you in?: ")
if(grade.lower()=="freshman"):
print("You are in Grade 9.")
elif(grade.lower()=="sophomore"):
print("You are in Grade 10.")
elif(grade.lower()=="junior"):
print("You are in Grade 11.")
elif(grade.lower()=="senior"):
print("You are in Grade 12.")
else:
print("Wrong input!")