If you need a software you can download Google Web Designer, if has this template already preinstalled and you can make some simple edits to create your own version.
Aside of the big ones (Navori, Scala, Onelan, etc) there was a guy promoting their solution in the sub, that was really impresive since it runs on RPI and can control many professional displays and some consumer TVs and they recommended to use web designer from google https://webdesigner.withgoogle.com/
let me recall the name of this solution
for your actual question: https://webdesigner.withgoogle.com/ or use gsap/tweenmax by itself.
>but it doesn't look like animations can be exported as a video?
yours is not a common usecase, so its not week supported. you may be able to use a canvas>video library, or a screen recorder or something.
in general, this is going to be using a screwdriver on a nail - you can kinda make it work, but its going to be a lot easier to use the right tool. paid, best bet is going to be after effects. free - gonna have to look around. blender or something else.
SVG with CSS animations. Anything with actual video will not compare to that in terms of load speed and bandwidth. Otherwise canvas animations are a good option as well. Keep it simple if you want good performance.
Using a HTML5 animator should help. Example: https://webdesigner.withgoogle.com/
Ideally you'd want to use as much code as possible and as little actual video as possible.
If you must use video, MP4 and WebM are your best options for video files. Also make sure they have no sound since those are suppressed by the browser anyways when autoplaying, and would only add to your file size.
"Google Toontastic" is a close match to Powtoon. You can find out more here https://toontastic.withgoogle.com/ - sadly it doesn't run on desktops but works with almost any other device.
For desktops, you can't go wrong with Powerpoint (PC or Mac) or Keynote (Mac + Free). Both support basic animations and transitions between slides. Recording is done by using any screen recorder software, preferably something that supports a microphone like Quicktime. Post editing for adding more audio tracks (background music) or cutting out unnecessary bits can be done with iMovie (Mac) or Window Movie Maker. Online tools like WeVideo for Education can perform this step just as well.
In the middle ground you have something like Synfig from https://www.synfig.org/ - a free animation package, compatible with Mac and PC/Windows. You're probably reaching the limits of what a grade 6 class can do.
Another free alternative I have played with is Google Web Designer (https://webdesigner.withgoogle.com/) - don't be fooled by its name. It primary purpose is animation, it looks and feels like a very simple version of Adobe Animate.
The GWD is the most advanced editor for HTML5 ads but if you are looking for an easy-to-use HTML5 banner editor you should definitely try BannerNow.
It has a simple timeline where each element has 3 animation sections (appear, stay, disappear) and you can use some pre-defined animations (left in, bounce, fade out, ...) or create your own animation presets and re-use them in future.
Google Web Designer was quite solid last time I used it. It allows to export as html+js pages and create interactive prototypes that reset or repeat. It's also used for google ads as far as I understand it so it's a useful program to know.
Honestly Dreamweaver might be somewhat sick, but I'm not about to start dropping cash for CC.... If you were looking for an IDE that is at the approximate level of abstraction as DW, maybe try https://webdesigner.withgoogle.com/.
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The other ones mentioned here don't really do anything to blend layout and coding, and are IMO a bit more low level.