Across:
8 Psychologist dismisses pigsty (‘psychologist’ dismisses ‘pigsty’ = scholo = SCHOOL) and in that there’s more than one lesson? (SCHOOL),
9 Most of (s) the amphetamines (uppers) (‘uppers’ without ‘s’ = UPPER-) chemically mixed (-CUT) (UPPERCUT) might floor one of those in Ring (UPPERCUT punch in boxing ring),
10 Carry on (carry on/manage = COPE) Constable (COP-) – the last movie (the last ‘moviE’ = -E) (COPE),
11 Drink (WATER) from a number springs (JUMPS) (WATER JUMPS) and, of course, the Arabs find them challenging (WATER JUMPS on a course challenge Arab/Arabians horses),
12 Promise (PLEDGE) by president (P-) to be brief (r) with accounts (‘ledger’ without ‘r’ = -LEDGE) (PLEDGE),
14 Massachusetts’ missing chest (‘Massachusetts’ missing ‘chest’ = massauts = SATSUMAS) produced by relatives of The Mandarins? (SATSUMAS),
15 Said almost simultaneously (IN THE SAME BREATH) without coming up for air (IN THE SAME BREATH),
18 Must (must) rearrange (= anagram indicator) (must = STUM-) half the accommodation (half of ‘B&B’ = -B-) for the French (the = -LES) (STUMBLES) trips (trips/falls = STUMBLES),
20 Character (LETTER) risked a removal from Kildare Street (‘risked a’ removal from ‘Kildare Street’ = ltreet = LETTER),
22 Decorator (PAINTBRUSH) suffering (PAIN-) from infectious disease (-TB-) in Dublin (-RUSH) (PAINTBRUSH),
24 Religious graduate (Bachelor in Religious Education = B-RE) hides hole (-O-) (BORE) in wet blanket (BORE),
25 State (= homophone indicator) spies (‘sees’ = ‘seas’ = SEAS-) from underworld (-HELL) (SEASHELL) picked up on ocean liner (SEASHELL on sand/ocean liner),
26 Short (e) story (‘lie’ without ‘e’ = LI-) and fables (-LIES) (LILIES) popular in the nursery (LILIES).
Down:
1 Reading material (SCROLL) for barman (Senior Counsel = SC-) on the breakfast bar (-ROLL) (SCROLL),
2 What you get with doughnut (HOLE in doughnut) that’s part of the course (HOLE part of golf course),
3 The perfect type (FLAWLESS) of chip in the dish, for example (FLAW-), from Takeaway (-LESS) (FLAWLESS),
4 For the most part, Old Skin (DUST) is taken in by beleaguered US Treasury (taken in by ‘beleaguereD US Treasury’ = DUST),
5 Staggers (staggers = trips) back (= reversal indicator) (trips = SPIR-T) around island (-I-) (SPIRIT) from the bar (SPIRIT),
6 Woman from Heidelberg (FRAU-) university (University of Limerick = -UL-) in Germany (Germany domain = .de = -D-E-) associated with good books (New Testament = -NT) (FRAUDULENT) that can be deceptive (FRAUDULENT),
7 Replace (SUPPLANT) Pulp’s (pulps) medley (= anagram indicator) (pulps = SUPPL-) with sound off (rant), missing intro (r) (‘rant’ without ‘r’ = -ANT) (SUPPLANT),
13 Cast aspersions on the sociability of (DEHUMANISE) person (-HUMAN-) in Waterford (Waterford = An Déise = DE-ISE) (DEHUMANISE),
14 Types of accommodation (SEMIS) on the summits of St Elias Mountains (on the summits of ‘St Elias Mountains’ = SEM-) on both sides? Igloos! (on both sides ‘IglooS’ = -IS) (SEMIS),
16 From which little acorns grow (acorns/tree nuts come in NUTCASES) for that mindless lot (NUTCASES),
17 It’s ideal for a quick getaway (BOLTHOLE) from 24 down (‘bolt’ = BOLT-) and 2 down (‘hole’ = -HOLE) (BOLTHOLE),
19 What the forward did over the head of the last man (goalkeeper) (forward LOBBED goalkeeper), as Rube Goldberg dismissed rugger (‘Rube Goldberg’ dismissed ‘rugger’ = beoldb = LOBBED),
21 Made (EARNED) to read (read) novel (= anagram indicator) (read = EAR-D) about leaders of Northern Europe (leaders of ‘Northern Europe’ = -NE-) (EARNED),
23 Angry (= anagram indicator) guy (guy = UG-Y) crossing the line (-L-) (UGLY) is not much to look at (UGLY),
24 Bar (bar/lock = BOLT) in Rush for the locals? (BOLT).