Level 56
Use WhereHas :
$students = Student::whereHas('user.customer', function ($query) use ($customer_id) {
$query->where('id', '=', $customer_id);
})->get();
It assumes Student has a belongsTo relationship with User named user and User has a belongsTo relationship with Customer named customer.
Also you should follow conventions for naming your relationships : lowercase name, singularized for belongsTo or hasOne, pluralized for *Many relationships.