Can I sit in sex education class as a Muslim?

It depends on what's taught and how. Learning the facts about puberty, the body, reproduction, health and personal safety is useful, and Islam itself teaches many of these topics openly. The concerns are explicit material and lessons that present sex outside marriage as normal. Many Muslim families attend the factual parts and, where the law allows, opt out of the rest and cover it at home.

Islam isn't shy about knowledge

Muslims sometimes assume these topics are taboo, but Islamic law openly discusses puberty, periods, wet dreams, ghusl (the full ritual bath) and marriage. A companion, Umm Sulaim, came to the Prophet ﷺ and said: "Verily, Allah is not shy of (telling you) the truth," then asked whether a woman must bathe after a wet dream. He answered her directly (Sahih al-Bukhari 130).

So knowing how your body works, how to stay healthy and how to protect yourself from abuse isn't against Islam. It helps you practice it.

Where the concerns are

Values: lessons may treat dating and sex before marriage as normal, which Islam doesn't.
Explicit content: some materials go far beyond what students need to know.
Modesty: detailed discussion in a mixed class can be uncomfortable. Islam values haya (modesty), and the Prophet ﷺ said that haya brings nothing but good (Sahih al-Bukhari 6117).

The Quran sets the frame that Muslim students can keep in mind during these lessons:

وَلَا تَقْرَبُوا۟ ٱلزِّنَىٰٓ "And do not approach unlawful sexual intercourse." Quran 17:32

What scholars and parents usually do

Scholars and Muslim educators differ in how cautious they are. Some advise withdrawing children wherever possible and teaching everything at home. Many others say to attend the factual, biological and safety content, while opting out of explicit lessons or those that conflict with Islamic values. Both approaches agree that parents should be involved and not leave it all to the school.

Know the rules

England: Relationships and Sex Education is part of the curriculum. Parents can usually ask to withdraw a child from sex education, but not from relationships education or the science curriculum. In secondary school, the student can choose to attend from about three terms before turning 16.
USA: rules vary by state and district, and many allow parents to opt out.
Other countries: in some, like Germany, opting out generally isn't allowed.

Most schools let parents see the lesson materials in advance. Ask for them, then decide calmly.

For students

If you're in the class, listen respectfully, take what's useful and don't feel you have to share personal views. If something conflicts with your faith, you can quietly disagree and ask a parent, imam or trusted Muslim adult about it later. For more on relationships, see Can I date at university as a Muslim?

Key points

Factual knowledge about the body is fine and useful.
Explicit content and anti-Islamic values are the concern.
Opt-out rights depend on your country.
Parents should teach these topics at home too.

Tip: Parents, ask the school for the lesson plans before the unit starts, and have your own talk with your child first. Children who hear it from you first are far less confused by what they hear in class.