error: passing ‘const Student’ as ‘this’ argument discards qualifiers

Не понимаю, в чём проблема с сортировкой.

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>


using namespace std;


struct Student {
    string name;
    string s_name;
    int d;
    int m;
    int y;
    int mark;
    vector<string> pref;
};

bool Comp(
        const Student& a,
        const Student& b){
    if (a.s_name != b.s_name) {
        return a.s_name < b.s_name;
    } else if (a.name != b.name) {
        return a.name < b.name;
    } else if (a.y != b.y) {
        return a.y < b.y;
    } else if (a.m != b.m) {
        return a.m < b.m;
    } else {
        return a.d < b.d;
    }
}


int main() {
    int n;
    cin >> n;
    map<string, int> univer;
    map<string, vector<Student>> st_in_un;
    int k;
    cin >> k;
    vector<Student> st(k);
    // мой код
    for (const auto& s : st) {
        for (const auto& u : s.pref) {
            if (univer[u] > 0) {
                univer[u]--;
                st_in_un[u].push_back(s);
                break;
            }
        }
    }
    for (const auto& [key, val] : st_in_un) {
        cout << key;
        sort(val.begin(), val.end(), Comp);
        for (const auto& s : val) {
            cout << '\t' << s.name << " " << s.s_name;
        }
        cout << '\n';
    }
    return 0;
}