I am a new comer to your website, and I absolutely love it. As a devoted Thomist (purely amateur of course), and avid reader of Fr. Reginal Garrigou-Lagrange, O.P., I find much pleasure in the disputations and web links on your site.
A quick question, and please excuse my ignorance if this is answered somewhere else on your site, or if the question is improperly posed:
What, if any, is the level of magisterial authority behind the 24 Thomistic Theses that were approved by the Sacred Congregation of Studies in 1914? Can any these be rejected outright today by a Catholic?
Welcome to Ite ad Thomam! Here is our beloved Garrigou's answer to your question (in his book Reality, Ch. 55):
What shall be the binding force of these theses? They are safe norms of intellectual guidance.  This decision of the Congregation, confirmed by Benedict XV, was published March 7, 1916.
The next year, 1917, saw the promulgation of the New Code, which  makes the method, the principles, and the teaching of St. Thomas binding on the professors and students both in philosophy and in theology. Among the sources of this canon the Code cites the decree of March 7, 1916.
Pope Benedict XV, on various occasions, expressed his mind on this point. He approved, for instance, in a special audience, the intention of P. E. Hugon, O. P.: to write a book  on the twenty-four theses. The author of the book  reports that the Pontiff said that he did not intend to impose the twenty-four theses as compelling internal assent, but as the doctrine preferred by the Church. 
1336 Proponantur veluti tutae normae directivae.
1337 Can. 1366, § 2.
1338 Les vingt-quatre theses thomistiques, Paris, Tequi, 1922.
1339 Ibid.: p. vii.
1340 P. Guido Mattiussi, S. J. had written already in 1917 a work of first importance on this subject: Le XXIV tesi della filosofia di S. Tommaso d'Aquino approvate dalla Sacra Congreg. degli Studi, Roma.
So there are four things being said:
1) The 24 Theses represent authentic Thomism--this is expressed in Pope S. Pius X's document that contains the theses (1914).
2) They are safe norms of intellectual guidance--this is expressed in Pope Benedict XV's decree in 1916.
3) The 24 Theses represent the preferred doctrine of the Church--this is expressed in the Pope Benedict's comment as reported by Fr. Hugon.
4) Thomism is mandatory for seminaries--this was expressed in the Code of Canon Law of 1917.
Whereas (1) through (3) are doctrinal truths, (4) is more of a disciplinary measure that depends on the Code of Canon Law (CIC). The old CIC was replaced by a new one (in 1983), which greatly mitigates the mandatory nature of Thomistic studies, reducing it to a vague recommendation. So, sadly, (4) is no longer the case. Therefore, a seminary professor could simply reject the doctrine of the 24 Theses nowadays without violating canon law.
However, (1) through (3) are still true. Of course, the Theses still (1) represent authentic Thomism, (2) they are still safe norms of intellectual guidance, and (3) they still represent the preferred doctrine of the Church. That was true then and it is true now. Not that one is a heretic if one denies these truths (for they are not de fide), but we do owe religious assent to them, so denying them would be an act of disobedient dissent.
a) My previous post, Can Non-Thomists be Good Catholics? (Pt. 1 of this post).
b) Pope Leo XIII's Aeterni Patris (on the Restoration of Christian Philosophy)
c) Pope Pius IX's Studiorum Ducem (on St. Thomas).