Why can't I sort this array

I am trying to figure out why my sorting code doesn't work. But I just can't understand this code shenanigans. I tried printing every array out but some random number appeared. I don't know anymore. Number I printed was variable j which should go over 20 but somehow printed out 27,28 and it ruined the sorting code, because it worked fine on other projects. Please code god help.you can see i and j printed out followed by numbers in that array, and you can also see the ghost number that came out nowhere(27,28,29).

#include<iostream>
using namespace std;
int main(){
    int sc[20], tt, min, max;
    
    while(true){
        for(int i=1; i<=20; i++){
            cout<<"Student "<<i<<"'s score : ";
            cin>>sc[i];
            if(sc[i]>40){
                cout<<"\nError\n"<<"Try again\n\n";
                break;
            }
            tt=tt+sc[i];
        }
        break;
    }
    
    cout<<"Average = "<<tt/20.0<<endl;
    
    //sorting
    for(int i=1; i<=20; i++)
    {       
        for(int j=i+1 ;j<=20; j++)
        {
            if(sc[i]>sc[j])
            {
                   tt=sc[i];
                sc[i]=sc[j];
                sc[j]=tt;
            }
        cout<<i<<" "<<j<<endl<<" "<<sc[i]<<endl<<" "<<sc[j]<<endl<<endl;

        }
    }
    //min max
        cout<<"Min = "<<sc[1]<<"\nMax = "<<sc[20];
        
        
}