These days most of the drawing tools that support a wide variety of diagram types will include some software engineering models in their collections. Commonly these are paid tools with a free mode that is limited in some way, such as the size or number of diagrams that you can develop. Typically they can export diagrams in a variety of picture formats, but not in e.g. XMI for import into another tool. On the plus side, their interfaces will be familiar to anyone who has used drawing tools before. On the minus side, because they are fundamentally drawing tools, it can be unfortunately easy to produce diagrams that look like UML (say) but are not actually syntactically correct UML.
Some of the web-based drawing tools worth considering are
- Draw.io https://app.diagrams.net/ (particularly popular with students after I invited them to look around and recommend online tools that were easy to use)
- LucidChart https://lucid.app/ – paid, but has a free mode that is enough for simple teaching purposes for example.
- Mermaid Live Editor https://mermaid.live/ is a live editing environment for diagrams using the Mermaid.js JavaScript library: diagrams are rendered as native SVG-based graphics by the browser itself. Be warned that the UML is approximate, e.g. wrong arrow-heads abound.