From Teflpedia

Th-alveolarisation involves the substitution of sibilants /s/ and /z/ for the dental fricatives th sounds. So that "thing" sounds like "sing" and "these" sounds like "zees".

It is not generally used by native English speakers and is considered a mispronunciation, albeit a common one for English language learners to make, particularly if their native language is Chinese or French. Native speakers may alveolarise some dental fricatives in rapid speech (e.g. an appeal in cricket "how was that?" is transformed by excited players to "howzat?" /ˈhaʊˈzæʔ/), but these tend to be both (1) part of rapid speech and (2) in the middle of phrases.

It may help students' listening if they are aware that other English language learners do this.

Some students can pronounce th properly, but find it difficult and can be a bit lazy and alveolarise th as they know they will be understood. Positively encourage such students to apply themselves and make the correct sound, as doing so does make them sound much better.

Words that have both /θ/ and /s/ in can be a problem for students, e.g. "something", "thesaurus", "photosynthesis".

Students who can produce the dental fricatives may hypercorrect sounds which should be /s/ or /z/ to /θ/ or /ð/, which makes them sound like they have a lisp, which may make them sound worse. These students need to recognise the spelling that the {th} digraph fairly reliably indicates a dental fricative.