Formal:
* She is absent from her lectures.
* She is not attending her lectures.
* She is skipping her lectures.
Informal:
* She's ditching her lectures.
* She's blowing off her lectures.
* She's not showing up for her lectures.
More specific:
* She's been missing lectures recently. (Implies a pattern)
* She's missed a lot of lectures this semester. (Provides a timeframe)
* She's not able to attend her lectures due to [reason]. (Explains the absence)
To avoid ambiguity:
* She misses attending her lectures. (Focuses on the emotional aspect)
* She regrets missing her lectures. (Highlights the consequence)
The best alternative depends on the context of your sentence. Consider who you're talking to, the tone you want to convey, and the reason for her absence.