Madonna is willing to "fight to the end" for custody of her son Rocco.
The singer's 15-year-old son walked out of her 'Rebel Heart' tour in December to stay with his father Guy Ritchie in London and Guy is said to have hired lawyers as he prepares to battle to retain custody of the teenager.
A source told PEOPLE: "She is going to fight, fight, fight, and she is going to fight to the end.
"She is not happy. She is an excellent mother, and people can say whatever they want - but her children mean the world to her."
Rocco is now living with Guy, his wife Jacqui Ainsley and their three children, Rafael, four, Rivka, three, and Levi, 18 months, in the UK and Madonna is said to be worried that Rocco is missing school.
Explaining he has missed some school, a source said: "He should have been home already.
"Madonna is a devoted mother, and a big priority is her children's education."
It was recently claimed the final straw for the teenager was when Madonna - who also has children Lourdes, 19, David, 10, and Mercy, nine - confiscated his mobile phone because she thought it was interfering with his homework.
A source said: "Madonna is quite firm with Rocco and she took his phone from him when it interfered with his studies.
"She is stricter than Guy and Rocco doesn't like it. When she took the phone away, it pushed him over the edge."