腰の入ったすけべ連中が多いから

Question

I have these lines from an old game, and wish to know what 腰の入った means. It is clear to me that it is some kind of fixed expression, because the parts do not make sense by themselves.

赤マントってＳＭ好きの変質者かなぁ。でも、高校生のうち
からあんなのに走るのかしら？　ああいうのって刺激が物足
りなくなって走るような気がするんだけどなぁ・・・あっ！

ということは、男子生徒じゃなく大人の犯行！？・・・・・
いやいや、うちの学校には腰の入ったすけべ連中が多いから
断定できないわよぉ～・・・・・

An attempted translation of the last sentence is "Ah, but this school has quite a few 腰の入った perverts, so I can't be sure."

I searched the Information Superhighway for this phrase, but mostly came back with documents discussing punching technique. ("腰の入ったパンチ")

Here are the lines from the game as images in case I have accidentally mistranscribed them:

Answer 1

This 腰の入った comes from 腰を入れる, an idiomatic set phrase that means "to put one's back into something".

腰を入れる

２ しっかりした心構えで事に当たる。本腰になる。「―・れて環境保全に取り組む」

So 腰の入ったすけべ連中 is people who put much effort into perverted acts.

Answer 2

「うちの学校{がっこう}には腰{こし}の入{はい}ったすけべ連中{れんちゅう}が多{おお}いから」

「腰を入{い}れる」 is an idiomatic verbal expression meaning "to set to work in earnest", "to give all one has got", etc.

Thus, 「腰の入{はい}った」 is the adjectival form of the above expression.

My own TL of 「腰の入ったすけべ連中」 would be:

"devoted perverts", "hard-core perverts", etc.

So, we have:

"Because my school has a whole lot of hard-core perverts, ~~~~"