Is abstaining from prayer when parents are absent, or intentionally abandoning prayer without a valid Sharia excuse despite being able to perform it, considered Shirk (polytheism) or Kufr (disbelief)?

A Muslim must observe prayer in all circumstances, seeking the countenance of Allah, not to appease a parent or anyone else. Allah Almighty says: "Guard strictly your prayers, especially the middle prayer; and stand before Allah with humility" (Al-Baqarah 2:238), and "So woe to those who pray [But] who are heedless of their prayer" (Al-Ma'un 107:4-5). The preponderant view is that whoever sometimes abandons prayer and sometimes performs it is not an apostate whose disbelief takes him out of the fold of Islam. This is the view of the majority of scholars, whereas the Hanbalis hold a less favored opinion that whoever abandons prayer altogether is a disbeliever.