Assuming it is an indoor wall and there is no water leak issue (paint not bubbling)
Scrape off will be tough since its old paint. Unless you're stronk like body builder.

I recommend to get a power sander to strip everything off to bare cement plaster.
One problem with old paint is that it sticks so good to the wall when you try to scrape off.
But the old paint rips off the moment you try to roll on new paint.

Once kena new paint, the solvent of the new seeps into the old paint and loosens the old paint from the wall. As paint is also sticky enough (its designed to be sticky to adhere to the wall) it rips off (by combination of chemical - solvent and physical - you rolling the roller) and the entire roller is fully coated (fxxxed up) with old paint. And now the roller is gunked up and you have to clean the whole thing. New paint is costly to be used as paint remover :lol:

If wanna try to just roll sealer over existing, IF the same thing happens (gunked up roller) with the sealer, ultimately still need to sand off or strip off the old paint already.

Bosch power sander available at 130-140 bucks. Will create iraq desert storm.

The other alternative is need to buy lots and lots of paint stripper chemical.. but after buying ten tins already can buy the bosch sander machine. This one less dust but toxic fumes issue.

Then use sealer (affordable water based one will be ok if its not severe issue).. if very powdery or a lot of whitish stuff come out by itself then consider using oil/solvent based sealer (much more expensive) as you might have to treat the alkalinity problem as well.

After that can paint over with finish coats.

To add on ceo684 reply above, if the chalk is on top of the old paint surface and the old paint film still binds well to your wall, then can just wipe off the powder on surface, let it dry and apply solvent based styrene-acrylic wall sealer before your next coat of finishing.

Do take note if your existing paint is solvent base then have to check on the type of the paint used to avoid solvent attack, if existing paint is water based then no problem. The only downside from this  is the pungent smell produced from the solvent.

Maxilite, Jotun, Nippon etc. Price diff few ringgit.

Maxilite can also be used for outdoor. Is economical.

Dont expect other normal wall paint to last longer than special formulated outdoor paint. Is called weathershield for a reason.

Any cheaper normal wall paint can also be used for exterior but are not able to withstand harsh weather condition.

For me, is either economical or weathershield. In between is a waste of money.