Let’s first look at examples in which the verb 置く is used in a literal sense.
The following sentences all mean different things as indicated in brackets.
- 台にテレビを置く。[habitual or future action]
- 台にテレビを置いている。[current state]
- 台にテレビを置いた。[past action]
- 台にテレビを置いていた。[past state]
They can be converted into the following noun phrases.
- テレビを置く台 [habitual or future action, or static property (in this case, purpose)]
- テレビを置いている台 [current state]
- テレビを置いた台 [past action or current state]
- テレビを置いていた台 [past state]
Of these, the one with the た-form (#3) is somewhat special in that it can also be understood as describing a current state just like the one with the ている-form (#2). However, it doesn’t quite sound so natural as, say, 太った猫, possibly because having a TV set placed on it is not so much a permanent property of the table as being fat is of the cat. It seems more natural to explicitly describe it as a state with the ている-form (#2) if that's what you mean.
When 置く is used in a figurative sense as in 重きを置く, it loses part of its quality as an action verb and this seems to blur the distinction between the two sentences below.
Outside of limited contexts in which it is understood as describing a future action, the first sentence is no longer about an action but a current, or permanent, state of mind of the people running the school. Personally, the second still sounds more natural, though. This could be precisely because the possibility of 置く referring to a future action cannot be ruled out completely.
The following pair in the past tense also means practically the same thing as one another.
Although I would still choose the second if I have to choose one, the naturalness of the first sentence seems to somewhat increase compared to the first sentence in the present tense above. Even if 置いた is understood as a past action, the state that resulted from it is already a thing of the past. Then, it is not much different from what the second sentence says.
As for the noun phrases, the following three are all correct.
The version with the た-form (#3) sounds more natural than テレビを置いた台 probably because the described property of the system is more permanent. The version with the ている-form (#2) sounds slightly less natural than the other two to me because it makes the described property seem like a transient state.