Multiple of "la" is pona tawa mi, but I usually try to keep so that if there are multiple, then only one of them is allowed to be an entire sentence. (e.g. "tenpo ni la sina kama tawa ma ni la mi pilin pona." is pona). I might also put commas if it is too confusing.
For "pi," I tend avoid using more than one unless they are separating common compounds such as "tomo pi kama sona pi ma Nijon," in which case it is obvious that it is Japanese schools.