4 letters:
buel, buhl, cool, dual, dule, fool, fuel, juel, jule, kool, mule, pool, poul, raul, reul, ruel, rule, tool, tuel, yule

5 letters:
-cule, abdul, adule, babul, baiul, beule, boole, boule, brule, buehl, capul, coole, crool, cruel, deuil, dhool, dhule, doole, dreul, duell, ghoul, goole, grool, hoole, huile, jewel, joule, keule, khul', kreul, muehl, noule, pecul, poole, poule, rahul, raoul, reule, rhule, roule, ruhul, saoul, scool, shool, shoul, shule, skool, smule, spool, spule, stool, thule, toole, truel, tuell, tuyle, utsul, vipul, yeule, youll, yuill, zeuhl

6 letters:
alcool, as-30l, babool, baoule, befool, boulle, bruehl, cerule, choule, curule, dobule, ethule, ferule, frioul, g.soul, gueule, gurule, hyrule, infule, insoul, jambul, kaboul, kagoul, lemuel, lobule, locule, lucule, luelle, macule, mccool, muelle, nabeul, nepool, nucule, omtool, oscule, perule, pilule, recule, retool, school, schule, scrool, shmuel, tooele, tuille, uncool, unfool, vocule, who'll, you'll, yuille

7 letters:
aktuell, ampoule, arghool, barbule, bascule, bouille, bulbule, cagoule, caracul, carpool, ciboule, corcule, damfool, drogoul, edicule, ethtool, fanduel, ferrule, festool, feuille, fistule, guayule, hangeul, jambool, kagoule, karakul, kurnool, laycool, makhoul, maqbool, medjool, misrule, o'toole, outfool, pethuel, pistool, precool, pustule, rantoul, rrdtool, saccule, sandeul, schuele, sertule, sporule, sproule, sproull, spruell, spruill, texpool, tilleul, tomfool, unspool, valvule, vanpool, zebrule

8 letters:
aedicule, agrifuel, air-cool, amygdule, b-school, barstool, blastule, canicule, car-pool, cesspool, chacmool, chamisul, chevreul, clausule, claypool, corneule, coronule, cryptool, deadpool, deepcool, deschool, digitule, dindigul, dzhambul, entitule, exajoule, exiftool, floccule, floscule, gag rule, genepool, girlpool, glenpool, gwenpool, heeltool, intitule, ipsambul, istanbul, j-school, kab-500l, linalool, lodicule, lovefool, metarule, molecule, monocule, neekolul, opercule, opuscule, overrule, pedicule, plantule, radebeul, radicule, reticule, ridicule, sesspool, silicule, spherule, spruiell, tidepool, tim pool, top-tool, traboule, ullapool, unbefool

9 letters:
arbuscule, audiotool, blackpool, bloemkool, campstool, caroncule, chondrule, claypoole, cysticule, dromgoole, edge tool, faldstool, farigoule, fascicule, follicule, footstool, foot rule, gameduell, glomerule, graticule, grembiule, hand tool, hierodule, home-rule, home rule, intercool, kab-1500l, karnivool, kilojoule, lawn tool, lenticule, liverpool, machinule, majuscule, megajoule, minuscule, monticule, motorpool, multitool, nanojoule, outschool, parannoul, pedascule, pellicule, petajoule, petiolule, picojoule, piquepoul, planetule, playskool, poeticule, pontypool, preschool, propagule, ramuscule, rookstool, self-rule, steenkool, stepstool, supercool, teurgoule, tire tool, toadstool, ulcuscule, undercool, vestibule, welshpool, whirlpool, wurbagool

10 letters:
animalcule, april fool, art school, board rule, boerenkool, canalicule, closestool, corpuscule, crepuscule, cuttystool, day school, fenestrule, freedstool, frithstool, girlschool, gribouille, hartlepool, highschool, homeschool, labor pool, law school, millijoule, minischool, motor pool, multi-tool, old-school, old school, plastidule, playschool, plumb rule, poiseuille, power tool, pre-school, realschule, slide rule, somnambule, spinnerule, step stool, tinkertool, vanderpool, vanterpool

11 letters:
afterschool, biomolecule, brit school, close-stool, creepschool, cutty stool, dame-school, disseminule, garden tool, golden rule, ground rule, groupuscule, high-school, high school, home-school, interschool, music stool, piano stool, prep school, that you'll, tokyo ghoul, wading pool, warrnambool

12 letters:
after-school, amuse-gueule, anonymuncule, apostolopoul, bellefeuille, berufsschule, bishop-stool, choir school, conciliabule, cutting tool, dance school, endoplastule, george boole, grade school, hearsay rule, hemispherule, languagetool, les claypool, machine tool, millefeuille, music school, night school, nonzero-rule, numbers pool, portefeuille, semifloscule, shaping tool, trade school, work-to-rule, working rule

13 letters:
chevrefeuille, church school, crazysexycool, dental school, ducking-stool, ducking stool, flying school, gram-molecule, gram molecule, infant school, intellektuell, junior school, macromolecule, majority rule, middle school, milking stool, mille-feuille, normal school, out-of-school, peter o'toole, prison school, public-school, public school, quatrefeuille, reform school, riding school, snipping tool, sunday school, supermolecule, supramolecule, swimming-pool, swimming pool

14 letters:
dancing school, driving school, grammar school, medical school, nursery school, nursing school, primary school, private school, sabbath school

15 letters:
annette o'toole, boarding school, business school, catholic school, cyril abiteboul, dipole molecule, graduate school, language school, linguistic rule, lord of misrule, training school

16 letters:
carpenter's rule, composite school, entrenching tool, grammatical rule, parochial school, protein molecule, religious school, secondary school, technical school

17 letters:
christopher poole, elementary school, exclusionary rule, historical school, old palace school, veterinary school, vocational school

18 letters:
best evidence rule, engineering school, junior high school, preparatory school, secretarial school, senior high school

20 letters:
comprehensive school

21 letters:
correspondence school

22 letters:
the god of high school

23 letters:
secondary modern school

25 letters:
korea under japanese rule

26 letters:
olympic-size swimming pool