Mongoose: cannot insert/find records - node.js

Question

I am new in mongoose and I'm facing some problem in it.
After running the following code, neither success nor error message is shown. And when I browse the webpage, there is no response.
Is there any problem in my code?
const http = require("http");
const mongoose = require("mongoose");
mongoose.Promise = global.Promise;
mongoose.connect("mongodb://localhost/node_tut", { useMongoClient: true});
const StudentSchema = new mongoose.Schema({
first_name: { type: String, required: true },
last_name: { type: String, required: true },
class: { type: String, required: true },
class_num: { type: Number, min: 1, required: true }
});
let Student = mongoose.model('student', StudentSchema);
let peter = new Student({
first_name: "Peter",
last_name: "Chan",
class: "1A",
class_num: 2
});
peter.save((err) => {
if(err){
console.log("Error inserting Peter");
return;
}
console.log("inserting Peter");
});
let mary = new Student({
first_name: "Mary",
last_name: "Hung",
class: "1A",
class_num: 6
});
mary.save((err) => {
if(err){
console.log("Error inserting Mary");
return;
}
console.log("inserting Mary");
});
const server = http.createServer((req, res) => {
// Send the HTTP header
// HTTP Status: 200 : OK
// Content Type: text/plain
res.writeHead(200, {'Content-Type': 'text/plain'});
console.log("request");
Student.find({}, (err, students) => {
console.log("result founded");
if(err){
res.end(err);
return;
}
res.end(students.toString());
});
});
server.listen(3000);