I want to get all the group(key) in my spinner then after that school no(key) into another spinner from firebase database : I'm a beginner & currently in a learning phase, please help me with this problem

This post might look like a duplicate to you but it's none of the previous questions had this much keys.

I want to get all the `group no 1` in my spinner then after that all the `school no` of group no 1 into another spinner from firebase database

Here's my database

[![enter image description here][1]][1]

here's my code
>java

    public class Collection extends AppCompatActivity {
    
        TextView ttl, tgat, tsch;
        Button btnshw, btngt;
        Spinner sping;
        DatabaseReference d2ref;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_collection);
    
            tgat = (TextView) findViewById(R.id.texigat);
            tsch = (TextView) findViewById(R.id.texisch);
            //tcls = (TextView) findViewById(R.id.texiclass);
            ttl = (TextView) findViewById(R.id.texirs);
            btnshw = (Button) findViewById(R.id.bshow);
            btngt = (Button) findViewById(R.id.bgat);
            sping = (Spinner) findViewById(R.id.spingat);
    
            btnshw.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    d2ref = FirebaseDatabase.getInstance().getReference().child("2018-19").child("Gat No 14")
                            .child("School no 109").child("Standard 2nd");
                    d2ref.addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                            String rs = dataSnapshot.child("Rupees").getValue().toString();
                            ttl.setText(rs);
                        }
    
                        @Override
                        public void onCancelled(@NonNull DatabaseError databaseError) {
    
                        }
                    });
                }
            });
    
            btngt.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
            d2ref = FirebaseDatabase.getInstance().getReference().child("2018-19");
            ValueEventListener eventListener = new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    final List<String> gatno = new ArrayList<String>();
    
                    for (DataSnapshot dsnap : dataSnapshot.child("Gat No 14").getChildren()) {
                        String gat = dsnap.getKey();
                        gatno.add(gat);
                    }
                    ArrayAdapter<String> adapter = new ArrayAdapter<String>(Collection.this, android.R.layout.simple_spinner_item);
                    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
                    sping.setAdapter(adapter);
    
                    /*I wasn't able to get the value in 1st spinner so I didn't
                     wrote program for second spinner*/
                }
    
                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {
    
                }
            };
            d2ref.addListenerForSingleValueEvent(eventListener);
                }
            });
        }
    }

Please help me with this.

  [1]: https://i.stack.imgur.com/kFb7u.png