Figure-27.5 shows the total energy level diagram for p-orbitals interaction.
Note that s-orbital is the lowest in energy due to maximum overlap.

Figure-27.5

Note:
Electron can occupy molecular orbital that are lower in energy.
Molecular orbitals may be empty or contain one or two electrons.
For two electrons occupied orbitals, electrons must be spin paired due to Pauli exclusion principle.
When occupying molecular orbitals, electrons occupy separate orbitals with parallel spin before pairing.
The highest occupied molecular orbital is known as HOMO.
The lowest unoccupied molecular orbital is known as LUMO.